how do get my sim unexpelled

Well, I haven't found that option in Sim Info Cheats section, but no worries, I can still help.

If you want your sim's expelled trait to be removed, use this cheat:

traits.remove_trait trait_HSExit_Expelled

Be sure to remove it just in case. Otherwise should your sim graduate with this trait, they'll still be treated as someone expelled and has to get a diploma from online to not have their career choice be restricted.

I'm not sure if it removing this trait will put your sim back in school (I'd guess not), but here's a cheat to do so:

careers.add_career career_Teen_HighSchool

Alternatively, do check the Find Job panel. Sometimes one of the available careers is High School.

I did try that a while ago, but they could just reenroll for the day, cause at the end of the school day they’d get expelled again…..