The reason why is because you probably were using some kind of cheat. If you use a cheat, the game will stop sending your sim to work and sending your sim to school. The only way to fix this is to delete the app, download it again, and start all over without using cheats. Or you could just create a new sim on your game and send them to work and for your pretense, you can purchase a desk and do cram or study to bring up their grade. Hope this helps!