Lack of Permanent Expulsion Tracking

Summary

Expelled students can re-enroll after being removed, which defeats the purpose of the expulsion mechanism.

Vulnerability Details

The contract only removes the student from the isStudent mapping in expel():

There is no tracking of previously expelled addresses, allowing expelled students to call enroll() again.

Impact

This allows malicious or previously removed users to rejoin the system without restriction, violating business rules.

Tools Used

Manual code review.

Recommendations

Introduce a permanent expulsion flag:

Update expel():

And enroll():