I agree the 6502 would be a nice choice, but I'd be more tempted to go with a modern RISC architecture like MIPS or ARM- MIPS is extremely clean and simple, but

I agree the 6502 would be a nice choice, but I'd be more tempted to go with a modern RISC architecture like MIPS or ARM- MIPS is extremely clean and simple, but it has the additional advantage of being something students are more likely run into in the wild someday.