I am not in the US - our schools use a spread them out and then have groups within the class at least for the early years. It just seems to make so much work with a wide range of skills in the class. Also when there are 24 kids in the class who have had between 4 and 22 months schooling I find it highly improbable that they fall neatly into 4 or 5 reading levels so some kids must be in groups above or below their ideal level.
They usually don't fall neatly into a few reading / math / science levels, you're correct. It takes a master teacher to be able to differentiate effectively with such a wide range of students, however, master teachers aren't the ones being hired, they're too expensive for the public school's budget, they're hiring the teacher with the min. qualifications that is the cheapest....and in many states all learning abilities must be mixed into the normal class room.
