Stanford University has been offering online courses for a fee for 20 years now. After free online course trend has started to increase they started to offer free courses as well. You can find the same courses offered here on edX and Coursera.
Click on the link on the left and go to the site. Read through the upcoming page carefully. You will see lots of options. Choose what you like though pay attention to course descriptions. As mentioned above some courses are not free. Pay attention to that.
Most High School level courses
are not free
Pictures from the Campus of Stanford University