UPDATE: Coursera is no longer free, usually. But, as I write during the quarantine, it is free, as a way to help others out. University and college students who enroll by July 31, 2020, will have free access through September 30, 2020. For non-college students: “To help everyone keep learning, we’ve extended the offer to earn a free certificate in 100+ courses through July 31, 2020.”
At Coursera, dozens of universities offer hundreds of courses ranging from guitar to chemistry, from physiology to language, and from art appreciation to business management. The courses begin at a certain date and run for a certain length of time. The student learns at his or her own pace, but assignments are collected. Certificates are given for completed courses (but no college credit). And..all the courses are free!
The courses may be taken by anyone over 13 years of age, but are created for the college level students, so they would be best for those who are in upper high school.
Use them as an excellent sample of college work load for your high schooler, or to complete credits, or even to increase your own knowledge!