I've heard of people doing this and getting expelled. I don't remember if it was for mining or whatnot, but the idea of surreptitiously installing software and harvesting is usually frowned upon. I've even heard of IT members losing their jobs over it.

Is it ethical? I don't know if I can be a good judge here. Ethics are essentially personal questions. You obviously have worries about its ethicality, so it sounds like you are already convinced its a bad idea.