A little background. I'm a librarian by trade, and word has gotten around that I'm experienced in the RPG hobby. A library in a neighboring county has created a successful D&D program with over 20 teenagers participating on a biweekly basis. The problem is that their staff member/resident DM/etc. has resigned his position. So now I've been asked to teach others on their staff how to DM (many the first time playing the game) to be ready in a couple weeks. I'll be there to help transition the club to new DMs, show them how to run, etc.
Has anyone been in a similar situation? Or do you have words of encouragement? Or just want to laugh at the predicament of the resident ENWorld pessimist who regularly laments "I'll never be able to run an in-person game of D&D ever again."
I have done this; it can be tricky but incredibly rewarding. I've run groups of 8-14 year-olds.
Assuming that the whole horde is there at once-
Split the players into teams of 3-5. One of them is the Caller. It is their job to find out what the other PCs are doing and tell you. This allows you (the DM) to receive input from 3-5 people instead of 12-20. The position of Caller, and the people on the various teams, changes each session.
Passive checks are your friend, use them liberally and call out the team that makes the discovery rather than an individual. This allows the kids to feel some reward for builds but also allows for some shared glory. Keep track of those that expressed a specific desire for specialization and give them opportunities to shine. For example, I had one young lady who played a ranger with a winged cat friend (very Diana-esque). Because of her character description, I called her out specifically when the party found a mushroom faerie ring, and she was the one who could guide the party safely though the encounter.
Develop some plentiful buying tables. Have most of what can be bought be useful single or triple use items (
c.f. flask of boric acid solution- drives off giant insects and cures monstrous fungi attacks.) For the most part, girls like pets and boys like toys. Everyone loves fire lizards, of course, but that's the trend. Favorite pets were fire lizards, winged cats, shadow dogs, rolling rocks, and an eye on two chicken legs that could dance around. Bonus points for the toys to be combined in novel ways that the players can discover. I had a list of 12 pets and 20 one use items, along with about 4 pages of regular stuff.
I strongly advise against any PvP whatsoever. Maybe some duels if consequences can be agreed upon, but it never goes well in my experience. Cliques form in that large a group, be aware of trolls. Given them a change to play with, rather than against, the other players. Usually after a discussion pre- or post- game the desire to play with the group wins out over the desire to poke other people in the eye.
Sandbox style games are a poor fit. Too many different agendas along with a group of passive people. A choice of A, B, or C can work. Those that wanted a different route have their option next. Adventures run 2-3 session. They seem to get bored with the EPIC QUEST.
Happy to answer any questions you might have!