All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Just honoring the students who excelled with prizes and gifts -without any conditions [from them -i.e. the students] or prior knowledge -then there is nothing wrong with it. However, it seems that the problem on which you based your question is the issue of prizes given to the winners among the students based on a contest among them.
There is also no problem in this, according to some scholars, as long as the prizes are not from the contestants themselves but from other parties -which is the school administration.
Therefore, you should not object to the school's management for honoring the winners with prizes -even if this honoring is done after a competition.
As for the concealment of knowledge for which a person is sinful, then it is the Islamic knowledge that people must know about religion, and there is nobody else to teach them this knowledge, it is in this case only that a person is sinful.
Allah knows best.