You sound like a smart guy, so please read the whole thing (sorry its long) before you make up your mind, you obviously started this post because you are usually whether or not you're doing the right thing.
You want tell yourself that you're doing the right thing, thats its about justice and equal/fair play, that you are somehow teaching them a lesson that will benefit them in the future.
But you're really just doing it for yourself.
People cheat all the time in this world, they're only hurting themself. You won't be doing yourself any favour in this world, if you get worked up by things like this and impose your will on others. You're wasting your own time worrying about other people when you should be focus on self improvement. I'm a year 12 in top boy school in the country (NSBHS), and even there people cheat (they're only hurting themself), but i simple ignore them and these little distraction and focus on my own study and life, and now I'm doing excellently and in a happy place.
If you are truly an honourable person, you'll just mind your own business and focus on your own studies, and improve yourself in such a way, that you'll beat them even if they are cheating. This is true competitiveness.
You now only establish that the fact is you look out for yourself and probably can't be trusted. Next time mind your own business.
Final dvice: Ignore outer distraction such as other people, and get worked up about these things, focus on your own personal improvement, you'll get far in this world.