Have you considered playing around with MIT OpenCourseWare? There are full-fledged MIT first-year courses on there, complete with video lectures by preferred faculty, problem sets/readings, and exams, but there are also high school level classes that are more like samplers or topical, which might be fun to dip into.
https://ocw.mit.edu/high-school/Check out some of the courses developed for high schoolers by MIT students.
These classes (as well as the more conventional full MIT courses) are all self-paced, and you may choose to do or not do the assignments.
