You have confirmed my suspicions about these visits.
I think primary school children (who are always escorted in my local zoo) do gain something and generally show an interest in animals. However, in my experience, unsupervised groups of secondary school children don't. They might be interested as individuals but peer pressure takes over. Maybe teachers are used to their behaviour and don't realise the effect it has on other visitors and, more importantly, the animals.
I could say stop pretending that education is involved and take the children somewhere more appropriate, but I know this won't happen as Education is a legal requirement of British zoos, and they need the income. So I suggest that the CRB approved adults who came on the coaches with the children leave the restaurant and check up on their charges. And that identifiable zoo volunteers lurk in 'trouble spots'.
And I'll keep gritting my teeth and counting to 10. Oh, the joys of Other People's Children