What is going on in the school when the messenger arrives?

Just before the messenger arrives we learn that:

Apparently there had been some appalling act of justice recently inflicted

This is a potentially confusing statement in and of itself, but we have also recently learned that Ichabod Crane is holding a "ferule" or ruler in his hand, which would be used to strike misbehaving students. We also are told of "birch of justice" behind the "throne," which is Ichabod's chair, and are also told this birch is a "terror" to "evil-doers."
Given the emphasis on physical discipline in this passage, it appears someone has just been beaten by Ichabod with the birch switch. This is reinforced by the quiet in the schoolroom, everyone "intent upon their books" and a "buzzing stillness" in effect. Everyone is temporarily frightened into good behavior by the punishment. This shows that for all his effeminate charm with the ladies, Ichabod can punish harshly.
It is at this moment that the messenger, a negro on a half-broken colt, arrives to invite Ichabod to an event that evening at Van Tassel's home.