Well, there’s also turtles to program in Python (i think) and there is Scratch.

I guess there’s a real risk (…like 100%) that I overestimate the motivation students have

Definitely sounds like it. But a motivated teacher is a very good thing. Maybe you’re able to get that spark across to some of the students.

API-like to abstract away the low-level components

You can always have some extra assignments ready, just in case someone is curious and wants to do/know more. A room full of studens will have a mixed amount of knowledge, abilities and motivation anyways.

I’m most interested in resource-constrained embedded systems. I like the attiny10 a lot.

I also ate a few books and datasheets on the Atmel chips in my lifetime. Their design is well-thought-out and probably an excellent subject to learn the concepts about microcontrollers.

As of now I like the ESP32. It is ridiculously overpowered if you’re used to something like the ATtinies or old ATmegas. With (at least) 520kB of RAM, two cores that work at 240MHz (depending on variant) and very nice peripherals. Also WiFi connectivity is really useful. But it definitely adds to the fun if you programmed the more constrained (previous generation of) microcontrollers and know how spoiled you are and can feel like a supervillain wasting hundreds of kilobytes of memory deliberately. Or (ab)use some of the peripherals for things that wouldn’t be possible with the few timers available on the Atmel chips. Or do trigonometry at crazy frequencies for your robots, because you can handle 32bit floating point numbers. But I’d agree, that doesn’t teach you the same things if you can do floating point arithmetics for cheap and don’t know if calculating a square root is an easy or difficult thing to do. The STM chips also have nice peripherals. But I haven’t really fiddled around with those.

Definitely hope you’ll have fun being involved in that STEM program.