Complete code

src/main/java/Main.java

@@ -70,7 +70,13 @@ public class Main {
         boolean success;
         switch (choice) {
             case 1:
-                //TODO
+                success = school.registerStudent(username, password);
+                if(success){
+                    System.out.println(" Student registered successfully!");
+                }
+                else{
+                    System.out.println(" Username already exists!");
+                }
                 break;
             case 2:
                 success = school.registerTeacher(username, password);
@@ -103,7 +109,13 @@ public class Main {
 
         switch (choice) {
             case 1:
-                //TODO
+                Student student = school.loginStudent(username, password);
+                if(student != null){
+                    studentMenu(student);
+                }
+                else{
+                    System.out.println(" Invalid username or password!");
+                }
                 break;
             case 2:
                 Teacher teacher = school.loginTeacher(username, password);
@@ -153,7 +165,12 @@ public class Main {
     }
 
     private static void enrollInCourse(Student student) {
-       //TODO
+        System.out.println("\n Select a course:\n");
+        school.getAllCoursesDisplay();
+        System.out.println();
+        String courseName = scanner.toString();
+        Course course = school.findCourseByName(courseName);
+        student.enrollInCourse(course);
     }
 
     // ==================== TEACHER MENU ====================

src/main/java/models/Student.java

@@ -11,5 +11,30 @@ public class Student extends User {
         this.enrolledCourses = new ArrayList<>();
     }
 
-  //TODO
+    public List<Course> getEnrolledCourses() {
+        return enrolledCourses;
+    }
+
+    public boolean enrollInCourse(Course course){
+        if(enrolledCourses.contains(course)){
+            return false;
+        }
+        enrolledCourses.add(course);
+        course.addStudent(this);
+        return true;
+    }
+
+    public String getMyCoursesDisplay(){
+        if(enrolledCourses.isEmpty()){
+            return "You are not enrolled in any course yet.";
+        }
+        else{
+            StringBuilder sb = new StringBuilder("\n My Courses:\n");
+            for(int i = 0; i < enrolledCourses.size(); i++){
+                sb.append("   ").append(i + 1).append(enrolledCourses.get(i)).append("\n");
+            }
+            return sb.toString();
+        }
+    }
+
 }

src/main/java/models/Teacher.java

@@ -32,22 +32,26 @@ public class Teacher extends User {
         return myCourses;
     }
 
-    public void showMyCourses() {
+    public String getMyCoursesDisplay() {
         if (myCourses.isEmpty()) {
-            System.out.println(" You don't teach any course yet.");
+            return " You don't teach any course yet.";
         } else {
-            System.out.println("\n My Courses:");
+            StringBuilder sb = new StringBuilder("\n My Courses:\n");
             for (int i = 0; i < myCourses.size(); i++) {
-                System.out.println("  " + (i+1) + ". " + myCourses.get(i));
+                sb.append("  ").append(i+1).append(". ").append(myCourses.get(i)).append("\n");
             }
+            return sb.toString();
         }
     }
 
-    public void showStudentsInCourse(Course course) {
+    public boolean checkCourseOwnership(Course course) {
+        return myCourses.contains(course);
+    }
+
+    public String getStudentsDisplayForCourse(Course course) {
         if (!myCourses.contains(course)) {
-            System.out.println(" You don't teach this course!");
+            return "You don't teach this course!";
-            return;
         }
-        course.showStudents();
+        return course.getStudentsDisplay();
     }
 }

src/main/java/services/SchoolSystem.java

@@ -16,7 +16,30 @@ public class SchoolSystem {
     }
 
     // ========== Student Management ==========
-   //TODO
+    public boolean registerStudent(String username, String password){
+        if(findStudent(username) != null){
+            return false;
+        }
+        students.add(new Student(username, password));
+        return true;
+    }
+
+    public Student loginStudent(String username, String password){
+        Student student = findStudent(username);
+        if(student != null && student.checkPassword(password)){
+            return student;
+        }
+        return null;
+    }
+
+    public Student findStudent(String username){
+        for(Student s : students){
+            if(s.getUsername().equals(username)){
+                return s;
+            }
+        }
+        return null;
+    }
 
     // ========== Teacher Management ==========
     public boolean registerTeacher(String username, String password) {
@@ -44,12 +67,21 @@ public class SchoolSystem {
 
     // ========== Course Management ==========
     public boolean addCourseToSystem(String courseName) {
-        //TODO
+        for(Course c : allCourses){
+            if(c.getName().equals(courseName)){
+                return false;
+            }
+        }
+        allCourses.add(new Course(courseName));
         return true;
     }
 
     public Course findCourseById(int id) {
-       //TODO
+       for(Course c : allCourses){
+           if(c.getId() == id){
+               return c;
+           }
+       }
         return null;
     }