You make good points. Even though I don't work at Barcelona Zoo (or any other one I visit, for that matter!I'm a Visitor Engagement Volunteer at Chester Zoo and have been for almost a year now. Most of the time, guests are nice, polite and genuinely interested in hearing about the animals.
Most of my volunteering happened during school trip season; and I always ended up at "schoolkid" hotspots, so I ended up talking to a lot of kids. I love talking to kids; they're often really bright and it's fun wording facts in a way they'd understand (e.g. using Maurice from Madagascar as an example when talking about our aye-ayes).
With most school trips, the kids were as good as gold; at worst, they were a bit noisy. The rare cases of bad behaviour were easily corrected by me, with the help of the teachers/parent volunteers.
However, there was one exception, when I was volunteering in the aye-aye house. It's normally a two-man job, but the guy who I was supposed to be with hadn't turned up, so I was alone. About 20-30 kids (it was a school trip) came into the aye-aye house and started getting pretty noisy (well, they were excited). So, I had to try and calm them down one-by-one, which went just about as well as you'd expect; when I got one kid quiet, another would pipe up. So I just thought "I give up" and went to ask the teacher for help.
What was the teacher doing when this was happening? Why, she was standing, back against the wall, on her phone! When I walked over to her and asked her if she could help me get the kids quiet, she looked at me, shrugged (as if to say "not my problem"), and immediately went back to texting. I eventually got the kids calmed down; by addressing them as a group and telling them why the aye-ayes didn't like loud noise. To their credit, they calmed right down and were good as gold the rest of the time.
I didn't file a complaint for three reasons. First of all, I couldn't see the uniforms (I'd only just gone in), so I didn't know which school it was. Secondly, I didn't know if the "teacher" was an actual employee (she might have just been a parent enlisted to help on the trip). The last reason was I didn't want the kids thinking I was mad at them; for these kids, I think it had been a long time since anyone in uniform was patient with them.
I have an "uncle" (he's my aunt's partner) who's a deputy head teacher and, when I told him about it, he was amazed I didn't file a complaint; if one of the teachers at his school had behaved like that, he'd have fired her.
Sorry if I sound grouchy here.
This is why I like working with kids more than adults sometimes; their minds aren't fogged up by nonsense social norms and bad reporting!