Jisho

中止 is way too formal for that kind of usage. You'll want to use やめる instead. Something like ○○はやめたいのですが。Keep it simple.

I'll throw in some additional information too:
中止 is used for things like contracts, plans, negotiations, sales of an item, sports matches, that kind of thing.
When you use something like やめさせる in your context above, it sounds like you are asking your parents to let you withdraw from a class or something. Which if that's your intended meaning then you should use やめさせる. But if you're trying to tell a receptionist or something that you want to withdraw then you shouldn't use やめさせる.