Imagine

I want you to stop and imagine the person you love the most. That person you just really care about. Now I want you to think about how you'd feel if you found out they were cutting their wrists so deeply their arms wouldn't stop shaking. What if they made themselves throw up to be pretty. What if they didn't eat for so long they collapsed? Would you make them your punchline? Make a joke about how you're gonna go cut yourself or kid about how someone should stop eating to lose weight?
No?
Well then why do you do that to other people's person they love most?
What is so funny about mental illness when it isn't someone you're close to?
Nothing. It isn't something to joke about. Those few laughs you might get, aren't worth that person in class feeling ashamed or stupid because you think them wanting to die is a laughing matter.
If it was that person who matters to you, if it was you, it wouldn't be a joke. So it isn't when someone else is in that situation. Just realize that.