What I've done now is they are no longer officially 'quittable.' Instead, you do a sort of cheat to bypass their quittable status and quit anyway. Previously, the way it worked is I made the school 'careers' 'quittable', which allowed you to quit/join them through the 'job' interface, but also meant that when taking on extracurricular activities, you had to choose that or school. Version 2.0 comes with some changes to address a longstanding nuisance about quitting and joining, which I hope will make the mod more enjoyable to use.
Optional Addon: Download (Simfileshare) | Download (Patreon) | UnlimitedPTO: Taking a day off school doesn't reduce the number of days off you have, so you can do it as much as you want. Latest: v2.0.1, released after EA Patch, July 20, 2021