As crazy as it sounds I swear by having a hair and make up artist (or skilled staff member on hand), for when you are working with chosen kids for set shots. It acts as a holding area for when it is their time slot, but you may be running late (inevitable) and they can check the kid’s hair is tidy, check for illegal jewellery and makeup, removing anything inappropriate. Supervising a shoot is a busy day running between current shots and upcoming shots, and it is easy to miss the small details so have someone checking the kids while you are looking at the bigger picture stuff.
I have had great shots ruined by messy hair and illegal jewellery. And you do not want your audience focusing on the wrong thing in a shot.
Also have a tool kit on site with hairspray, comb, brush, bobby pins, school ribbons, elastic bands, lip balms for dry lips, makeup wipes, face powder for shiny faces, tissues.
Also great for when you are taking shots of teachers i.e. close ups, in case their makeup needs a touch up. No one likes a shiny face!
And to be honest a recent reliable alum could do the job because they have such great makeup and hair skills these days!