In principle, you are entitled to the same wage as regular employees in the company. This is the minimum wage set by the collective labour agreement of the sector the company belongs to.
If you do overtime, then you should be paid extra. In fact, overtime worked on Sundays and public holidays must be paid at double the rate.
Your wage should be clearly stated in the student agreement. Or at the very least, its calculation should be explained on paper. The agreement should also specify when you will be paid.