<p>I think that 'high school students" refers to students who will be continuing HS the next year. Many private colleges have summer programs geared towards HS students, not incoming freshmen.
The UCs probably allow incoming freshmen to start in the summer because it’ll reduce the strain on classes during the rest of the year and allow students to graduate in a timely manner.
If you want to take classes over the summer, take some at a CC that can transfer as GE credit because that’ll open up more space for majors/minors once you’re at USC.</p>