I had this problem too - I assume it's because the base game only allows 8 Sims per household, and you need mods to increase that number (I use MCCC). What I ended up having to do in my 10-person household was go to household manager, split the family into two (I did 7 and 3, but 8 and 2 should work as well - just have to make it so that the households are 8 or under), and then you can send them to university. I put the Sims I wanted to send to university in a household temporarily (you can use freerealestateon if you don't plan on having them there long term and don't want to spend simoleons on this temporary fix), then use the computer to send them to university (this is assuming you want to move them into a dorm - I don't remember, but I think I couldn't get it to let me move them into a dorm from the manage worlds screen, I think I had to use the computer once they were already living in a private dwelling).
The downside is that once you do this, you might have to jump through some hoops if you want to move any of those Sims back into the original household if it would make the original household larger than 8. (Personally, my households usually only go over 8 through 'Try for Baby'/babies being born. I never bothered trying to do it any other way.)
