What does "lift their skirts ... and grab the eyeglasses right off someone's face" mean?

From Gavin De Becker, The Gift of Fear: Survival Signals that Protect Us from Violence (1998):

"That's true," she said. "Every day I have to predict what the kids will do, and I succeed for reasons I can't explain." After a thoughtful pauses she added: " But I can't predict the behavior of adults."

This is interesting, because the range of behavior children might engage in is far, far greater than it is for adults. Few adults will suddenly throw something across the room and then break into uncontrolled laughter. Few women will, without apparent reason, lift their skirts above their heads or reach over to the next desk at work and grab the eyeglasses right off someone's face. Few adults will pour paint on the floor and then smear it around with their feet. Yet each of these behaviors is familiar to substitute teachers.

What is the meaning of the sentence placed in bold?

My impression of the excerpted language is that the author is reflecting on the fact that children often behave in ways that adults do not, and yet children's behavior is somehow easier to respond to and manage than adults' behavior because children's behavior has an element of guilelessness or a simplicity of motive and action that makes it easier to calm or defuse.

As for the portion of the sentence that the poster finds baffling, "lift their skirts above their heads" means precisely what it sounds like: a four- or five-year-old child wearing a skirt may suddenly decide to lift it up over her head, for no explicable reason other than that it suddenly struck her as an interesting or fun thing to do.

Likewise, "reach over to the next desk at work and grab the eyeglasses right off someone's face" refers to a situation where a child is sitting at a school desk next to someone else sitting at a school desk, and the first child suddenly reaches over and takes the eyeglasses that the second child is wearing off his or her face. Again, the motive is obscure but may well be nothing deeper than the first child's sudden impulse to take them off the second child or to try them on himself or herself.

The author includes the words "at work" in this phrase to emphasize that in a work situation involving adults, no such behavior would be at all likely to occur—and if it did, it would be rather shocking—whereas in the context of a children's classroom the behavior may be deemed naughty or inappropriate, but it is hardly shocking (to the author, anyway).

The meaning of those phrases is their literal meaning. If you do not understand any of the words, you can look them up in a dictionary.

The passage is saying that children will, when faced with a substitute teacher, misbehave. Examples of such behavior include lifting one's skirt above one's head (if one is wearing a skirt, presumably) and removing one's neighbor's eyeglasses.