When I was teaching programming my go-to was the JavaScript canvas api. It’s 2d only, and very simple. And being on the web once a student has made something we can host it for them and they can show their friends.

I have a ~20 line html harness which sets up a page with a full screen canvas element and gives you global window width & height variables. That’s all you need to get started. And it’s real JavaScript - so students learn a useful programming language as a result, and the advanced students can go nuts and add sprites, sound and networking if they really want to.