Is it permissible to study at a school where the father has acted as guardian over his children to boost their grades or facilitate their admission into specializations, while the son fears the same might happen to him?

It is permissible to study in this school as long as you have not sought favoritism from the teachers, but rather asked your father not to interfere with their evaluations. If some of them still show you favoritism despite that, then there is no sin upon you, and they bear the consequence of their action.