What is the ruling on students doing school assignments for payment?

It is not permissible for a student to submit work done by someone else, because that constitutes cheating, which is forbidden by Islamic law. The purpose of assignments is to train the student and measure their level. Assisting a student in cheating is forbidden. Sheikh Ibn 'Uthaymeen mentioned that students hiring others to prepare research and dissertations for them contradicts the university's purpose and is a form of betrayal. There is no harm in preparing work for others for a fee if the instructor explicitly permits it.