I'm on the fence with all this. If done correctly, I don't think its dangerous to pull the power. One could argue that if the first time the student lands(even if the CFI is correcting often)with a stopped prop isn't an actual emergency, this is a good thing. And I'd agree with that. However, if ANY little thing goes wrong, I mean like a mid-air in the pattern while coming down or anything of the sort, and the student dies or is seriously injured, and the Feds find out the engine was turned off by the CFI, regardless of it having anything to do with the accident...good luck.