Make a copy of your save game and then try moving the family out and back in. This will probably remove all your teens from boarding school but you can send them back.
You can also try resetting the sim with the cheat resetsim firstname lastname (name of your SIm)
Yeah, my teen went missing while in boarding school (icon totally is gone from household). She still shows up in the relationships area of her family members, but they can't chat to her or anything, only text. I tried resetSim [first] [last], but she did not show up. I evicted them and then moved them right back in (from edit town mode) and they were all right back where they started. Re-enrolled the teens in boarding school, and it's been working fine ever since 🙂
