It would really be too late for any kind of fix (DebugEnabler's Fix Homeworld, evicting and moving back in, replacing the school, etc.) to be of any use for the same day where school has either already started or is about to start. The fixes more typically take hold the next school day.Or the next school day after they register, if re-registering for school is required. I would consider stalling this sim's aging (are you using NRaas SP?) or subtracting a few days from their total age using MC Cheats until you get this sorted out so that they don't lose half or more of their remaining school time.
I had a teen once who, all story related not really due to glitches, didn't get to go to high school for almost two sim weeks in the beginning. There was a move to a new (to them) world by way of the Traveler mod, then there was a family reunion/wedding he needed to be off for, then a holiday, then he got sick, though nothing serious (Vector mod). One of the benefits of playing much longer age spans is that it there is usually still plenty of time to catch up when things like this happen though it did take a bit of work and effort on his part at the time. :)