"Charles knows. I'm not just going to sneak out in the middle of the night." He'll at least take his leave in person. He owes them all that much, after everything that's happened. He wants Charles to know that things are okay between them, for the moment.

He wants Raven to know something too. "I don't intend for it to be permanent. It won't be another ten years before you see me again. I promise."

This is a promise he intends to keep. He'll let Charles have his school, and stay out of that, but mutants need to stick together, and he'll be there when they need him. Probably sooner than that, too, but there's more than just the school building that he needs to fix.