Minimum number of days in a week for all students shall do the trick but I could not find such constraint for students but there is one for teachers.

Hmm, yes, you touched a sensitive point

It is very difficult to add this constraint implemented correctly (perfectly) for the students, such as we did for the teachers. It might be easier to implement it not perfectly, but I would not like to add such a thing in the official FET.

I will add your suggestion in the TODO (students min days per week).

Implemented perfectly means to take care of the interaction with the other constraints.