I was a choral singer for five+ years (and still would be if I had a program outside school to sing in) and although I'm female and didn't experience the same in voice changes as you do, I was around male singers enough I understand what does happen.

I assure you that this is a pretty natural thing for males who also sing. When you're singing, your vocal chords are working in different ways than when you simply shout or are speaking normally.

Though you may have been able to sing higher notes before, your voice may be changing in a way that makes it more difficult for you to hit them now without some warming up, some training, and some practice. It may also mean that in your singing life you're going to be better fit to sing baitone or bass, rather than tenor (you didn't state what you sung now, sorry to assume, that is if your school seperates into parts like that, as well). Not to mention the vocal range and part you have now isn't necessarily going to be the vocal range and part you have in a couple years.

Also, because you are male, you have a great fortune that us women singers do not: falsetto. All men have a falsetto, you probably know what it is, but just in case, it's the higher range male singers can sing after the end of their normal vocal range. A lot of male singers don't know how to properly use their falsetto, because to first start using it, it can take kind of a push a lot of them don't necessarily want to take.

I'd recomend you go speak to your choral director, hopefully if they're as good as choral directors I've seen in my choral life, they can help you work on your vocal range by singing a part that is comfortable to you and teaching you how to reach and use your falsetto to sing higher notes.

Hope this helped and wasn't too jumbled an explanation!