Why is the combined second/third grade class such a big deal in S2?

Why is it such a big deal that Ms Schemmenti has a combined class of two different year groups? Is that not a thing in America? In my country most classes are made up of two year groups (sometimes even three) and it has very little effect on anything? Every time I watch that storyline I'm wildly confused about the two year groups being split down the middle and it being such a huge palava. Is it actually such a oddity to have two year groups in a class or is it solely to do with Melissa's reluctance to ask for help?