Can principals and teachers be evaluated fairly with yearly KPI?

Let say a new principal of a prestigeous top secondary school or junior college decide to try some new methods to train the students. As a result, the school's results drop a little, maybe no longer top, but still within top ten. After five years, the school's board call up the principal and said, "Ever since you took over, our school is no longer the best. Although we know that you emphasise on training the whole person, not just for academic results, but our school ranking has dropped and so you have to go. Maybe you should go to those school where the students are not so gifted and try your methods." Let say twenty years later, it turned out that most of the leaders of the country turned out to be from that school during the principal's term of office and they credit their success to him (or her). How shall we reward this principal, since the proof of his success takes twenty years to mature?