Illinois is an "at will" state. That means that you are hired 'at will' and they can fire you for any reason at all, or no reason. There are a few reasons that are illegal, like your age, gender, religion, etc. But staying away from those things, they can fire you at any moment just because they don't like your looks.
It seems unfair, but you know you can also quit for any reason or no reason.
If they hired you at 16, they must have KNOWN you wouldn't be able to work late on school nights. It's dumb for them t make an issue of it now.