Question

._. Peter is very lazy. He is the (active) student in our class. (??????????????)

Answer 1

he is the least active student in our class

Answer 2

It seems like you have a sentence that might need some clarification or revision. Let me try to help you make it more clear:

"Peter is very lazy, yet he is the most active student in our class."

Is this the intended meaning you wanted to convey, or did you have a different idea in mind?