I'd recommend a BBC emulator. Simpler than the C64 (in my opinion!) and there's a wealth of technical reference material available.
Granted, there's plenty available for the C64 too, but hey ;)
If you wanted to stick with 6502, perhaps a Vic-20 emu, rather than C64? It would steer people well away from graphical and sound niceties and would concentrate their minds on the programming.
Another alternative is an Amiga emulator - though there are serious legality issues there (you'd need DevPac assembler, or similar, to teach constructively).
Granted, there's plenty available for the C64 too, but hey ;)
If you wanted to stick with 6502, perhaps a Vic-20 emu, rather than C64? It would steer people well away from graphical and sound niceties and would concentrate their minds on the programming.
Another alternative is an Amiga emulator - though there are serious legality issues there (you'd need DevPac assembler, or similar, to teach constructively).