I'm going to tell a classmate that they don't work hard enough
What will you tell them about their work ethic?
I'll tell them that they are a serious slacker
It seems your classmates should be able to do as they please. You should not stick your nose in your classmates business. It just causes drama.
Ill tell them that they are slacking and should have higher expectations.
If you truly want to tell your roommate this than you really need to make sure you address it in a helpful way that is not condescending.  They need to know that you are only bringing it up as a way to help them out, and because you ultimately care about them.