I use notebooks a lot in teaching, and when the students are interacting with them, I will often have closed-up cell groupings with hints, or answers, to problems I had them first try to work out themselves. It will all be under a subsection or subsubsection heading cell called something like "Hints", and above that I put "Don't open this until you have tried to solve it yourself!"

In live classes, I would like to go a step farther and 'lock' these sections so that I can control when the students open them (so we can do each step of an exercise without them seeing later material which would give the game away). Given all the scope for programmatic notebook manipulation, I'm sure there must be a way to have, say a text input box that responds to a passcode and opens the next closed-up cell grouping. But this is not my forte, so I'm seeking suggestions!