It's indeed difficult to do good education research, and instructors prefer to just mess around without a control group instead. But the intellectually humble thing to do would be to admit that the experiment can't differentiate between a method that turns bad students into good ones and one that merely identifies good ones through selection effects. When more conscientious researchers get around to doing a proper randomized controlled trial on an intent-to-treat basis, it invariably turns out to have been the latter.