If you want to look at hardware which might also be suitable for programming in C/C++, the BBC micro:bit might be a good alternative to an arduino. It has a more self-contained set of peripherals, and is programmed in ARM assembler. It uses drag-drop style programming (you need an ASCII format hex file) over USB. Of course, you need to set up a tool chain to compile the code, unless you want to write raw hex by hand (or analyse compiled code from the online tool chains). Halting debug (single step and modifying registers) is supported, so it's a capable platform to work on if you want hardware.
In terms of a compiler for Cortex-M0 (as in the micro:bit), the educational toolchains support an in-browser compiler which demonstrates that it's not actually too complicated.
Of course, there is a fair amount of initialisation required if you want to actually turn the LEDs on or look at the push buttons (let alone the SPI accelerometer on board). You might include some assembler within an online compiler C++ project - that ought to work (if you use the ARM mbed toolchain rather than the school specific toolchains).
An alternative is to look at something more low-level, using a verilog simulator (but there is non-trivial cost involved in the simulator which might put this out of reach to schools - it's more targeted at universities who are teaching embedded hardware, and start-ups). The whole Cortex-M3 processor and a supporting system can be downloaded from ARM for free. This runs code (and has a model which prints out disassembly as it runs). - disclaimer - I developed this product.