Within project ReleQuant, I developed an online learning environment about general relativity targeted at upper secondary school students. In January 2018, the Norwegian Centre for Science Education launched the program on the learning platform Viten. You can find the Norwegian version of the program here.
Despite our better knowledge, physics in schools is still taught from an outdated 19th century point of view. I think students should learn about curved space and warped time from an early age. Why? Take a look at my article on Titan.