A clean RTOS for the Raspberry pi 3 and 4?

Oh well, then I’ll forget freeRTOS. Bit of a pity that since the RPi is in a lot of teaching environments
as so pupils and students could learn how a RTOS system works - and compare it so Unix applications.

Please give an example of such boards that can run freeRTOS, I need the power of a RPI, one can of course use a RPi 3 which uses a lot less power.