Answer: It is already in affirmative form.
Explanation:
When using affirmative form, you are confirming that subject in question does indeed possess a certain quality. For instance, "Parrain is a prolific answerer" is affirmative because it is saying something that Parrain is.
"Pedro is a good student" is therefore already affirmative.
If you wanted to write it in the negative which means that the subject does not possess the quality in question, it would be:
"Pedro isn't a good student".
