I'd say that if you include kids that are not ready in advanced classes one of two things is going to happen. Either the slower kids are going to get so left behind that they won't be getting anything out of the class or the class will get slowed down to the level of the slowest kid, so the higher level kids won't be getting much out of it. I don't know how you could make a winning situation out of it - the teacher won't have time to teach to all the levels...