This answer addresses US usage only; other Englishes may employ these terms differently.
Your instinct is right: pupil student doesn’t work, because just as in ordinary speech all institutions of learning, of whatever level, are referred to as ‘schools’, learners at all levels are referred to as ‘students’. Pupils, though not uncommon in professional literature, is now rather old-fashioned in ordinary speech and tends to be reserved for primary or middle-school students. I think most US high-school students would feel insulted to be referred to as ‘pupils’!
[Scholar won’t work either. That now means an academic expert and is no longer used in the US for ‘students’ at any level.]
It is not at all unusual for high school students to take university or college courses, and the most common term for this is dual enrollment—but this of course designates the situation rather than the student. As far as I can tell, there is no word or phrase in general use to speak of the student. This research report, for instance, uses the cumbersome “dual enrolled high school students”, “dual enrollment participants” and “students completing dual enrollment”, and gets fourteen pages in before employing its shortest term, “dual enrollees”.
If you need to use this term just once, to indicate the status of a person or character, I’m afraid you’re going to have to spell it out:
While still in high school Ryan took courses in IT at the local community college.
If you need a term for repeated use you’re still going to have to spell it out at the top, but you can introduce a brief term of your own there for subsequent use. I would go with “dual enrollees”, or something on the order of “DE students”:
In this paper I will examine the experiences of high-school ‘dual-enrolled’ for credit in college courses. Such ‘DE students’ frequently encounter difficult social problems alongside the academic challenges they face. ...
Some things just don’t translate.