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

You have to use 'là' in the sentence has this structure : Subject + là + complement (noun). But you can't when complement is an adjective.

"Tôi là sinh viên". That's right. :)

" Tôi là sinh viên " is right