I'm going to tell a classmate they smell
What are you going to accomplish with telling them that?
He asked me to take a shower after being so smelly
Smelly people are the worst to be around, but you need to be more tactful. If it is out of revenge then it won't even be helpful. Make sure you are sincere and trying to actually help them in being better at hygiene.
I can see you are a person who smells too.
I don't think it's a good idea to call people out like this, don't you think they would get embarrassed by it? How about kindly telling them by handing them a body spray? They would get the idea and appreciate the gift too.