Saw it
Callum had caught the moment that Apryl and Keith kissed. It hadn't been intentional. He was actually on his way back with drinks thinking that the other two could probably use some extra water. Plus, he wanted to check and make sure that they were doing alright since they weren't used to the effects of the caffeine.
It just happened that this was the exact moment in time that Keith would lean down to kiss Apryl. At least from what the smaller werewolf could see, it didn't look like Apryl was against the kiss or disliked it by the fact that she didn't react harshly once it was over.
A lot of students knew who Keith was because of how polite and helpful he always was with everyone. It would certainly be a big deal to quite a few students who knew him if the news traveled as it was well known that he hadn't taken much of an interest in year and a half that he had been at the college.