I’ve worked in a museum, and school groups are the worst. Elementary kids are usually OK. They’re excited and don’t listen, but they generally keep their hands to themselves and like it when you pay attention to them. Kids in the 12-15 range, though, are awful in this kind of situation. I would generally keep my spiel to a minimum and try to position myself so they were as far from the more valuable items as possible. Expecting one museum employee to protect everything the kids could come into contact with is a bit much.
And for those of you who say the painting looks like someone just threw a bunch of paint at the canvas, that a three-year-old could do it, that it doesn’t take talent or ability. That’s fine if you have that opinion–just know that you’ll sound like an idiot when you say it and promptly be dismissed as a philistine. You don’t have to know anything about art to have an emotional reaction to it, but it helps to have a little bit of information before you try to explain why you don’t like it. And sometimes learning a little bit about the artist or the work will make it grow on you. Open minds and all that stuff, ya know?