Changing things so everyone is in college would be a major change, i don't know if it can be done...
The fact that there's College and HS is why there's a split between the Layla, Elena, Val group and the Erynn, Alexis, Iris group.
Plus things like the students being targeted by the colleges, with agents and recruiters following their progress.
Would Nyx making it more explicit that these are all seniors a few months from graduating(in addition to the classic assurances that "this girl is 18 years old") be enough for Steam?
Get them talking about college applications(which with them being seniors should be a topic honestly), plans for the future, how their HS experience ends in a few months, where they plan to go next year...