Would someone like to explain to me why this question "CPUs for retro computer school project" was considered off-topic?
The reason given was "Questions seeking recommendations for specific products or places to purchase them are off-topic as they are rarely useful to others and quickly obsolete."
The OP was not looking for a specific product, he was looking for an architecture that was appropriate based on a set of criteria (external bus, > 50 MHz, serial interface).
Now his idea of retro is a little different than mine (I was thinking 6502, 600 etc.) but I guess a 25-year old architecture (MIPS) is enough retro for today's kids. So I recommended the PIC32 family.
Uh duh, I don't think the PIC32 is going to be "quickly obsolete" (in fact, Microchip just brought a whole new family, the PIC32MZ which I mentioned and the OP is considering when it becomes available). So this whole thing about "rarely useful to others and quickly obsolete" is nonsense in relation to this question. I think my answer could be very useful to someone else searching the site.
Now I admit, I did suggest, as an example, a specific PIC32 for him to look at, and where to buy it. If you want, I can take out my recommendation, and leave it more general. But I don't see why that is necessary, because questions are closed based on the wording of the question, not one of the answers, am I right? Or do I have that wrong?