From personal experience, I'm obliged to say an obnoxiously clichéd phrase; "the older they are, the worse they get.". In general, younger children, who are accompanied by teachers at all times, tend to be better behaved than the older children who are generally allowed to roam the zoo freely with little to no supervision.
I see it all the time at Barcelona zoo, and at the Cologne and Frankfurt zoo, which I visited at the same time as COUNTLESS school groups, it was the tweens and teens that made me want to pull my hair out. Confession; at the Grzimek Haus, there were a bunch of twelvies RUNNING IN CIRCLES around the house, laughing and yelling... So at one point I lost my cool and snarled "MUND HALTEN!!" At them. That shut them up...
Same in the UK. Primary aged school children are not the problem, they are always supervised due to young age etc. Secondary ones aren't, they are allowed to run free in the zoo. The teachers /supervisors who come with them often treat it like time out, convening in the cafeteria until its time to usher the kids back onto the coach transport back to wherever they came from.