This is a joke, I can't tell you or even know if anyone knows what everyone's surname is, it's never really come up. Most of the characters you're never told what they are, there's only a few that are known, so if no one ever calls someone by their surname, then why would you expect people to know it, after all, the MC has only been there one semester at this stage, after all, you don't find out the MC's surname until Sage asks him after he finds out that she is a Burke, but he knew her surname was Morgan, so he wouldn't bother to ask others what her surname was as he was under the impression it was Morgan.
As for Jill's surname, I don't think it ever comes up until it did, not one DIK told him, although I don't think they knew he was dating her until after the Halloween party, others, do we even know their surnames, I think the only ones we now know are the main LIs, I think even with the DIKs we only know the MC's, Rusty's and Derek's surnames, I couldn't tell you what Tommy's, John-boy, Leon, Jacob, Nick or Jamie's surnames are. Same with the HoTs, don't know theirs either except for Sage, Josie and Maya.