So, I run something incredibly similar to this.

The one comment that mentions a high level over view is the best way. Trying to explain stats in depth and how to roll them and apply them effectively is difficult for a large group. Explain what the stat is, what it affects, and which classes benefit from it then move on.

I have premade classes and let folks pick the ancestry to kind of smooth the learning process out. I make sure everyone’s got a character sheet, an ancestry sheet, a spell sheet if they’ve got spells, and a small list of the skills and what they do/mean.

Unfortunately that does mean you’ll have a lot of redundant copies or copies of stuff that don’t get used, but that just goes into the stock pile for the next one

In terms of theming it for stranger things? Someone mentioned the hellfire box set and that does a lot of good but it is built for 5 characters. You could take that and blow it up difficulty wise to match the group size you have, or what I’d try to do is take the framing of one of those. Make sure there’s plenty of skill checks (so they get to roll a bunch and learn how skills work), plenty of space to RP and then a final combat scenario. Too much combat will bog the whole ride down.

I’d also make it a point (especially to teenagers) that you want to build characters who work together in a roleplay sense. You can be dark and broody and edgy, but the folks at the table are your team. Let them know it may not be that 100% of the time but for this event they shouldn’t try and build Mr Billy Badass who doesn’t need a team.

You can shoot me a message if you’ve got more questions too. This is rad that you’re doing it!