Are you seriously suggesting that students in a college setting would normally use the phrase "next year" meaning the next calendar year instead of the clearly obvious meaning of next school year? Really?
Yes, I'd assume if she was a 3rd year she would say ''I'm graduating next semester" not "next year". So she's in her 4th year and will graduate months later.
Besides it wouldn't make sense for her to be a 3rd year, she was shown as president of HOTs in DIKs flashback where Tommy and Rusty were 1st years. Tommy and Rusty are now 4th years, so Sage should be too.
I don't get why Jill was at the preps mansion at the time, she's currently a 3rd year so she shouldn't be in college during the flashback. I assume she's applying or scouting the preps for a place to stay when she comes to the college next semester.