I had this issue a while ago where 3/4 kids would go to school and the other just wouldn't go. I used to just play it off as a rebellious kid but now I know that you can Shift click the sim in question and then ResetObject (DEBUG). You can also go to the cheats box (Ctrl+Shift+C) "testingcheats true" and then "resetsim firstname lastname" This will only work if the sim does not have any punctuation or spaces in their last name like St. Claire will not work and you will have to reset your sim by their ID number. But changing their last name in CAS to StClaire is easier and more helpful.
If this does not work, travel with the sim to another lot. Go back home and then wait a sim day. You can also save the game and that will instantly fix a problem for maybe a few sim minutes.
Also, there is a teen homework bug. So stop having your teens do their homework. Sell it off, make sure they can't touch it. Then instead of homework have them read a book for an hour every day. And use "careers.promote highschool" when you feel like they deserve to go up a letter grade.
And if you have mods, try clearing your cache.
