I have just spent the last hour or two working on a combination lock in Minecraft. The principle is sound, my idea works, it is just the darn redstone update glitches that are so common in multiplayer Minecraft that are causing me issues. So if I end up doing single player 'adventure' maps to teach the students things I have the perfect combination lock, or numerical answer question, or anything else that is order specific in terms of input. I will probably record a tutorial on it anyway as I think it is a cool idea. I have not worked on the cell since my last update so nothing interesting there. So I guess that just leaves screenshots of the combination lock. Enjoy.
I have spent a little more time messing around and I think I have it fully working, except for the glitches. All the incorrect numbers reset the system, also the 'correct' numbers in the incorrect order will reset the system too. Here are the updated screenshots, notice the added lines on the red wool in front of the pistons, that is the reset line. I think it works pretty well.
I am sure this could be neatened up considerably, and made much smaller, a project for another time though.