I don't think it has anything to do with you being a student, but rather with you not being employed full time. Say you were working for other companies part time as well and you went over the tax threshold then it will make sense as to why you are being taxed more. Full time employees can only earn from one source whereas part timers can earn a lot more from a lot of different places. Maybe speak to them and asked that they change your contract to full-time for tax purposes only?