(Not a Teacher)
As Trabb's son, who's hitting himself with a blue bag, approaches, the main character decides to keep a calm, unperturbed look on his face as he passes him. When they get close, however, the boy pretends to be afraid of him and jumps out into the middle of the street to make a scene.
Note: I've never read Great Expectations, so I can't offer too much insight into what's going on here. The kid sounds like an obstreperous brat, though.