I would say exploring the opportunities and interests.
This is the first time in their lives the kids have the actual access to the specific subjects: bio, chem, physics, lit, foreign language etc. (it’s all “science” and “social studies/language arts” and crappy boring math in Middle school anyway) It gives them the chance to see which subject they like best, what they want to study more.
Continue or re-evaluate existing interests and/or try something new (maybe a theatre should replace that violin or some volunteering work will be more fulfilling than a science olympiad?)
Another role of High school I see is to teach them working, studying at the college level, test if they are up to it.
And overall, if the school is doing its job, it should be more or less clear by the end of Junior year which kid will do well in a competitive college and which will try CC first and which will just go working without any extra intellectual challenges.