The phrase "John is the best output of Kaaa school" sounds somewhat awkward in English. "Output" is generally not used to describe people in this context. Instead, you could say: "John is the most outstanding graduate of Kaaa school." "John is Kaaa school’s most famous alumnus." "John is the best-known graduate from Kaaa school." "John is Kaaa school’s greatest success." Each of these alternatives conveys that John is a highly distinguished or celebrated person among the graduates of Kaaa school and would sound more natural to English speakers.

I would rather write it this way. "John is the most outstanding student in Kaaa!"