All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The medical certificate is intended to show that the candidate does suffer from any contagious disease and the like, which can affect his admittance in the university.
If this certificate was signed by the doctor without him examining the student, then this is considered fraud and deception, and this is not permissible.
The fact that some students did so does not make it permissible for you to do the same thing.
You can correct this by issuing a real medical certificate and submitting it instead of the first one, if possible. If this is not possible, then you get a medical certificate (undergo the required medical checkup) to make sure that you fulfill the condition (of being admitted). Thereafter, if you are accepted in the university, then there is no harm in joining it. However, you should seek the forgiveness of Allah for what you have done, regret it and be determined not to return to it again.
Allah knows best.