If you don't know him very well maybe you should get a bit closer to him - just talking to him as a friend. At least that way you will know him better and see for sure if he is really the kind of person you like and not just someone you decided to crush on "to crush on someone".
Once you are sure you really like him, you can write him a note if you don't know him too well still, or you could just say it to him, blurt it out, as long as there is no one else about. If you become friends, chances are you will be alone at some point when you will have your 'chance'.
Still, do keep in mind that in grade 6 there are probably many people who don't really feel prepared to have a boyfriend/girlfriend relationship so if you are thinking of asking him out as well as 'confessing' you like him perhaps you should think carefully to yourself "Do I really like him? Can I picture myself going out with him? Would it go well? Would I be allowed?".
I don't think he would be the kind of person to tell everyone if he did say, but if he does - he wasn't worth your time and you won't be seeing him again.
Also, if you tell him and he laughs at you or says something inconsiderate you could - and
should - point out to him that that was unnecessary. You can tell him that if you took the trouble to admit it he should not really be treating any 'admirers' with disrespect. Hopefully, that will not be the case and he will take it well.
Maybe you want to visualise the different outcomes before carrying it out, like a kind of 'mental preparation'.
Well, I'll stop typing, I think I just felt like talking a lot today.
I hope it isn't a too long read and is helpful.