I think a kid at my school is faking mental illness and I'm the only one who realizes it.
Are you sure you are the only one who can see the faking?
Yes, I'm the only one who's been around for a long time, so I know his tricks
It makes sense that if you're worried he is abusing the system that would upset you. You still shouldn't be making accusations of someone for something that others just don't think is true. Have you tried talking to this person to see what's going on with them?
I don't need to talk to him because I know he won't see reason or care about his fellow man. I just know he's a liar.
You can't just accuse someone you don't like. They could have a real medical diagnosis. Did you know not all illnesses have the same symptoms?