Oh, dear. This is quite the awkward dilemma! Honestly, though, you should be able to talk to your friends about this. I know, easier said than done, especially if they are not REAL friends, but just people you have to hang out with at school (because we all know that school would be a living hell if we absolutely had no one there to hang out with at all!).
If you absolutley can't talk to them honestly, you can either get sick that day, or, painfully, just grit your teeth and struggle through the day. Worst case scenario, you lose these three friends, but I highly doubt it would come down to that.