I mean, you should probably report this to your boss. Imagine that he finds out that they're doing this. Even if you didn't take days off or do the clocking in, it sounds like there's no plausible way you could have not noticed. He'll wonder why you didn't report this and probably not trust you, even though you didn't participate.

It's true that you risk retribution from the other employees if they were to realize you turned them in, but maybe there's a way that he can help you so that they don't realize you did anything, like decide he needs security cameras or something, which would show them if they were missing from work. He'd never have to mention that you said anything and they might all stop as soon as the cameras went up. (Friends boss did this a few months ago to catch someone they knew was stealing from work, but didn't have any evidence on yet).

There's also the chance that if you don't have evidence, you won't be believed and I don't know what advice to give you there. If you think your boss would listen to you, it's probably worth telling him. If you don't think so, this would be a lot riskier.