Tôi là sinh viên is correct. "là" is like "am" in the case that you are a noun, like singer, student, manager, etc. If you want to say that you are verb. like "I am happy" you would drop the là.
ex.
Tôi *là* ca sĩ.
I *am* a singer.
Tôi *là* bác sĩ.
I *am* a doctor.
Tôi buồn
I am sad