Alright! I managed to do all kinds of "tests" so I could be able to reply with the most accurate info as possible.
So long story short, yes the mods are what is making this glitch happen.
I started a family with one kid and I let him grow as a teen WITH mods. The sim as a kid didn't have any trouble being in school plus having an after school activity (boy scout) while having mods. However, once it turned into a teen and left the after school activity, and I had to re enter him in another one, it gave me a notice saying if I was sure i wanted to QUIT my current job (high school) to join this after school activity, bringing the aforementioned glitch.
I decided to leave it there, take off mods and restart the game to then join in the after school activity while also being in school, and it all worked perfectly. Then after a while I decided to check if I could dupe the game and re-enable the mods while the teen was in the after school activity, so I did! I enabled the mods and I restarted the game, but it gave me an error when I wanted to load up that safe file, telling me I had to restart the game or something (sorry I don't have the exact words, I immediately went online to answer this question)
So yes the mods are the source of the problem here, and only happens with teens.
I don't expect this glitch to be fixed, but I wanted to put an end to this question thread
thank you for replying 🙂