I'm not sure why they lock them in. Paranoia about keeping them on site, possibly, or about protecting the girls (who have two much larger rooms for the same number of people, and are allowed to come and go freely to the toilets).

When they do Holiday Orchestra at the same venue with similar numbers, everyone is trusted to wander around, and they have a great time. Perhaps the legal situation is different for a school activity, but I think they've just sent themselves into a blind panic over the whole thing of having taken what's basically your bog-standard end-of-term concert and turned it into a 'prestige' 'event' at a prestigious venue. Like much of modern life, it's all about image management and avoiding getting sued.