diff --git a/src/main/java/seedu/address/commons/util/StringUtil.java b/src/main/java/seedu/address/commons/util/StringUtil.java
index 61cc8c9a1cb..73cd5558624 100644
--- a/src/main/java/seedu/address/commons/util/StringUtil.java
+++ b/src/main/java/seedu/address/commons/util/StringUtil.java
@@ -65,4 +65,23 @@ public static boolean isNonZeroUnsignedInteger(String s) {
             return false;
         }
     }
+
+    /**
+     * Returns true if {@code s} represents an unsigned integer
+     * e.g. 0, 1, 2, ..., {@code Integer.MAX_VALUE} <br>
+     * Will return false for any other non-null string input
+     * e.g. empty string, "-1", "+1", and " 2 " (untrimmed), "3 0" (contains whitespace), "1 a" (contains letters)
+     * @throws NullPointerException if {@code s} is null.
+     */
+    public static boolean isUnsignedInteger(String s) {
+        requireNonNull(s);
+
+        try {
+            int value = Integer.parseInt(s);
+            // "-0" and "+0" is successfully parsed by Integer#parseInt(String)
+            return value >= 0 && !s.startsWith("+") && !s.startsWith("-");
+        } catch (NumberFormatException nfe) {
+            return false;
+        }
+    }
 }
diff --git a/src/main/java/seedu/address/logic/commands/GradeAssignmentCommand.java b/src/main/java/seedu/address/logic/commands/GradeAssignmentCommand.java
index fce7660d2e8..9985d1e6c5c 100644
--- a/src/main/java/seedu/address/logic/commands/GradeAssignmentCommand.java
+++ b/src/main/java/seedu/address/logic/commands/GradeAssignmentCommand.java
@@ -24,7 +24,7 @@ public class GradeAssignmentCommand extends Command {
 
     public static final String MESSAGE_USAGE = COMMAND_WORD + ": Edits the score of the assignment belonging to a "
             + "student by the student's index number used in the displayed student list, and the assignment's index"
-            + " belonging to the student."
+            + " belonging to the student. "
             + "The assignment will also be marked as Submitted.\n"
             + "Parameters: "
             + "[" + PREFIX_STUDENT_INDEX + "INDEX] "
diff --git a/src/main/java/seedu/address/logic/parser/AddAssignmentCommandParser.java b/src/main/java/seedu/address/logic/parser/AddAssignmentCommandParser.java
index 7867908c14f..1543b9269db 100644
--- a/src/main/java/seedu/address/logic/parser/AddAssignmentCommandParser.java
+++ b/src/main/java/seedu/address/logic/parser/AddAssignmentCommandParser.java
@@ -30,24 +30,24 @@ public AddAssignmentCommand parse(String args) throws ParseException {
             throw new ParseException(String.format(MESSAGE_INVALID_COMMAND_FORMAT, AddAssignmentCommand.MESSAGE_USAGE));
         }
 
-        argMultimap.verifyNoDuplicatePrefixesFor(PREFIX_STUDENT_INDEX, PREFIX_ASSIGNMENT_NAME,
-                PREFIX_ASSIGNMENT_MAX_SCORE);
-
         Index studentIndex;
         AssignmentName assignmentName;
         int maxScore;
 
+        argMultimap.verifyNoDuplicatePrefixesFor(PREFIX_STUDENT_INDEX, PREFIX_ASSIGNMENT_NAME,
+                PREFIX_ASSIGNMENT_MAX_SCORE);
         try {
             studentIndex = ParserUtil.parseIndex(argMultimap.getValue(PREFIX_STUDENT_INDEX).get());
             assignmentName = ParserUtil.parseAssignmentName(argMultimap.getValue(PREFIX_ASSIGNMENT_NAME).get());
             maxScore = ParserUtil.parseMaxScore(argMultimap.getValue(PREFIX_ASSIGNMENT_MAX_SCORE).get());
         } catch (ParseException pe) {
-            throw new ParseException(String.format(MESSAGE_INVALID_COMMAND_FORMAT,
-                    AddAssignmentCommand.MESSAGE_USAGE), pe);
+            throw new ParseException(
+                    String.format(MESSAGE_INVALID_COMMAND_FORMAT, AddAssignmentCommand.MESSAGE_USAGE), pe);
         }
 
         AddAssignmentCommand.AssignmentDescriptor assignmentDescriptor =
                 new AddAssignmentCommand.AssignmentDescriptor(maxScore, assignmentName);
+
         return new AddAssignmentCommand(studentIndex, assignmentDescriptor);
     }
 }
diff --git a/src/main/java/seedu/address/logic/parser/ParserUtil.java b/src/main/java/seedu/address/logic/parser/ParserUtil.java
index 19af243b541..3f060560deb 100644
--- a/src/main/java/seedu/address/logic/parser/ParserUtil.java
+++ b/src/main/java/seedu/address/logic/parser/ParserUtil.java
@@ -10,6 +10,7 @@
 import seedu.address.commons.core.index.Index;
 import seedu.address.commons.util.StringUtil;
 import seedu.address.logic.parser.exceptions.ParseException;
+import seedu.address.model.assignment.Assignment;
 import seedu.address.model.assignment.AssignmentName;
 import seedu.address.model.student.Email;
 import seedu.address.model.student.Name;
@@ -118,13 +119,10 @@ public static AssignmentName parseAssignmentName(String assignmentName) throws P
     public static int parseMaxScore(String maxScore) throws ParseException {
         requireNonNull(maxScore);
         String trimmedMaxScore = maxScore.trim();
-        int parsedScore;
-        try {
-            parsedScore = Integer.parseInt(trimmedMaxScore);
-        } catch (NumberFormatException e) {
-            throw new ParseException("The score must be an integer!");
+        if (!StringUtil.isUnsignedInteger(trimmedMaxScore)) {
+            throw new ParseException(Assignment.MAX_SCORE_MESSAGE_CONSTRAINTS);
         }
-        return parsedScore;
+        return Integer.parseInt(trimmedMaxScore);
     }
     /**
      * Parses a {@code String score} into a {@code int}.
@@ -135,13 +133,10 @@ public static int parseMaxScore(String maxScore) throws ParseException {
     public static int parseScore(String score) throws ParseException {
         requireNonNull(score);
         String trimmedScore = score.trim();
-        int parsedScore;
-        try {
-            parsedScore = Integer.parseInt(trimmedScore);
-        } catch (NumberFormatException e) {
+        if (!StringUtil.isUnsignedInteger(trimmedScore)) {
             throw new ParseException("The score must be an integer!");
         }
-        return parsedScore;
+        return Integer.parseInt(trimmedScore);
     }
     /**
      * Parses a {@code String studentIndex} into a {@code int}.
@@ -149,16 +144,12 @@ public static int parseScore(String score) throws ParseException {
      *
      * @throws ParseException if the given {@code studentIndex} is invalid.
      */
-    public static int parseStudentIndex(String studentIndex) throws ParseException {
-        requireNonNull(studentIndex);
-        String trimmedStudentIndex = studentIndex.trim();
-        int parsedStudentIndex;
-        try {
-            parsedStudentIndex = Integer.parseInt(trimmedStudentIndex);
-        } catch (NumberFormatException e) {
-            throw new ParseException("The score must be an integer!");
+    public static int parseStudentIndex(String oneBasedIndex) throws ParseException {
+        String trimmedIndex = oneBasedIndex.trim();
+        if (!StringUtil.isNonZeroUnsignedInteger(trimmedIndex)) {
+            throw new ParseException(MESSAGE_INVALID_INDEX);
         }
-        return parsedStudentIndex;
+        return Integer.parseInt(trimmedIndex);
     }
     /**
      * Parses {@code Collection<String> tags} into a {@code Set<Tag>}.
diff --git a/src/main/java/seedu/address/model/assignment/Assignment.java b/src/main/java/seedu/address/model/assignment/Assignment.java
index af23254253a..eb7dc46b646 100644
--- a/src/main/java/seedu/address/model/assignment/Assignment.java
+++ b/src/main/java/seedu/address/model/assignment/Assignment.java
@@ -15,8 +15,11 @@
  */
 public class Assignment {
 
-    public static final String MESSAGE_CONSTRAINTS =
+    public static final String MAX_SCORE_MESSAGE_CONSTRAINTS =
             "The maximum score should be greater than or equal to the minimum score of 0";
+    public static final String SCORE_MESSAGE_CONSTRAINTS =
+            "The score should be between (inclusive) of the minimum and maximum score";
+
     private static final int MIN_SCORE = 0;
 
     // Identity fields
@@ -36,14 +39,14 @@ public Assignment(Student student, AssignmentName name, int maxScore) {
         requireNonNull(name);
         this.student = student;
         this.assignmentName = name;
-        checkArgument(isValidScore(maxScore), MESSAGE_CONSTRAINTS);
+        checkArgument(isValidMaxScore(maxScore), MAX_SCORE_MESSAGE_CONSTRAINTS);
         this.maxScore = maxScore;
     }
 
     /**
      * Returns true if a given maxScore is a valid score.
      */
-    public static boolean isValidScore(int test) {
+    public static boolean isValidMaxScore(int test) {
         return test >= MIN_SCORE;
     }
 
diff --git a/src/main/java/seedu/address/storage/JsonAdaptedAssignment.java b/src/main/java/seedu/address/storage/JsonAdaptedAssignment.java
index e2e404742f1..c164df06d29 100644
--- a/src/main/java/seedu/address/storage/JsonAdaptedAssignment.java
+++ b/src/main/java/seedu/address/storage/JsonAdaptedAssignment.java
@@ -57,8 +57,8 @@ public Assignment toModelType(Student student) throws IllegalValueException {
         if (!AssignmentName.isValidName(assignmentName)) { // Assuming you have this validation
             throw new IllegalValueException("Invalid Assignment Name");
         }
-        if (!Assignment.isValidScore(maxScore)) {
-            throw new IllegalValueException(Assignment.MESSAGE_CONSTRAINTS);
+        if (!Assignment.isValidMaxScore(maxScore)) {
+            throw new IllegalValueException(Assignment.MAX_SCORE_MESSAGE_CONSTRAINTS);
         }
 
         final AssignmentName modelAssignmentName = new AssignmentName(assignmentName);