Is my school allowed do this?

I started out the year rough with a surgery and then a slip and fall caused me to miss the first month. I fell behind and received little to no help. After being pretty much being pushed out of that high school i attended a new school with the impression i can return to my old school the next school year. After attending the new school for a month or so, i had meniscus repair surgery and fell really behind. i then got little to no help(yup, again :P) in catching up, then i tried a homeschool which was to confusing the way it ran. I then tried to register for next year at my first school....then they tell me i can't cause i'm not caught up in credits. I know i dont have enough to be a junior, but i do have enough to be a sophmore again. they still say i can't go back with my credits, is this public high school allowed to do this?