What is the ruling on private schools that give students more [grades] than they deserve, and is it a sin for one to enroll in them?

You have not clarified what the mentioned schools give to the student that he does not deserve. Is it meant that they grant him high grades he does not deserve? If so, that would be forbidden deception (ghish), and it is not permissible for the student to accept such deception if he can refuse it. For someone who can only attend these schools, let him strive to avoid deception as much as possible. If some deception occurs without his will, then the sin is upon the one who committed it.