Challenge yourself on small coding exercises called "kata". Each kata is crafted by the community to help you strengthen different coding techniques. Master your current language of choice, or quickly pick up any of the 55+ programming languages supported.
Arcade Develop student programming skills by quickly creating and modding retro arcade games with Blocks and JavaScript in the MakeCode editor.
Micro:bit From dancing robots to banana keyboards, your micro:bit has all the features you need to create amazing projects—the possibilities are endless! In this website students can code and use the simulator to see their work or add a micro:bit. It has tutorials and lessons to use.
A physical micro:bit V1 or V2 is not necessary, but they are fantastic for increasing student engagement and physically seeing their code in action.
This site has many simulations helpful in understanding various computer science topics taught in high school.
The index has a list of all the topics taught on this site.
Some favorites:
CodeHS is a coding education platform with an LMS, various curricula and IDE sandboxes, and more. There's a free version, or you can pay for more features (like a built-in gradebook).
CodingBat is a free site of live coding problems to build coding skill in Java and Python (example problem).
CodingBat is a project by Nick Parlante, a computer science lecturer at Stanford.