Firstly, when cheating in school occurs, it takes the place of learning. The whole reason we study a topic is to learn. When you claim to have prepared for an assignment, but you turn in someone else's work as your own, for example, you aren't meeting the core goal of education. A student doesn't cheat and learn—it's one or the other.

Secondly, when you cheat, you're claiming that you are doing the required work even though you're not doing it. That's inherently dishonest. Your teachers expect the work you turn in to be the result of your own effort. When it's not, and you say it is, that's lying.