Is it permissible to attend private supplementary lessons imposed by some teachers, which are likely to be a bribe in exchange for obtaining deserved grades?

The action of the aforementioned professors is considered a betrayal of trust, and it is not permissible for them to link the giving of grades to students taking private lessons from them. If they do so, they are unjust, and what they take may be a bribe. As for the students, there is no sin upon them if they are unable to repel injustice and obtain their right except through this method.