"Sen öğrencisin."  (My attempt - You are a student)

"Sen öğrenci misin?"  (My attempt - You are a student, aren´t you?)

"Sen öğrenci değilsin  (My attempt - You are not a student)

I think the second one is ´are you a student?´

BTW the ´sen´ is not entirely necessary and is usually only used for emphasis.

Correction:

Sen öğrenci misin?  -  Are you a student?

(Sen öğrencisin, değil mi? - You are a student, aren´t you?)

The rest is correct.