Which is correct? And what is the difference between them?

A. What did you do after leaving school?

B. What did you do after you left school?

C. What did you do after you had left school?

With all my respect...

There is no difference in meaning. A and B are alternative ways of expressing this. C is a poor use of past perfect, which is unnecessary with the preposition 'after'.
They're the same, more or less. It's easier just to say "What did you do after school?", as 'school' implies 'the period during which you were in school as a student'.