He could make them into a self-aware interest group that is scared of him. Bullies understand force. What's that spell when you lift them upside down by the leg? I'd start by talking to Fred and George and seeing if they could get spells like that into a pre-packaged consumable form. And get some serious self defense training happening. Have there been any chapters so far on actual research based training methods for building skills? Or has Harry just been trying to 'clever' his way out of stuff?

A little crowd control doesn't seem like something Harry should have difficulty arranging.