Something I do with many of my own rockets, which I think would work well for group rockets.
I pencil in the alignment lines for the fins.
I cut “cheaters”, these are made of 1/16” balsa, cut 1/16” wide and the length of the fin.
I glue the “cheaters” on one side of each line (doesn’t matter which side, as long as it is the same for each of the three or four fins.). Because they are so thin and light, the go on really easy and stick, since there is no significant weight to pull them out of place. When putting them in place, I smear a bit of glue on the OTHER side of the line, since this will be the adhesion site for the fin (so at least 1/2 of the double glue joint is, well, glued.)
When it comes time to glue the real fins in place, you have a built in alignment guide that makes sure the fin is straight, plus you have more surface area for adhesion so the fin sticks better. You could go a step further and apply a thin coat of glue to the root surface of the fins ahead of time and let that dry too. (the other 1/2 of the double glue joint.).
Go much further than this and you are basically building the rocket for them, rather than letting them build it.