@laenen, I can only answer what I know, which are mostly pertaining to my daughter, or at least gifted/homeschooling. 
Dual enrollment course at Stanford I never heard of it, but at Berkeley is relatively easy and is often done, through UC Extension Open Enrollment system, as they don’t seem to really care about the student age past 13 (required legal age to make an online account) You have to pay full for the course though , even if you are a CA resident. An early college student told me that it was worth the money.