Glassings is just your everyday teacher who wants to overthrow the government. Nothing too out of the ordinary, right?
Okay, so Glassings isn't your everyday teacher. He is the one adult that Partridge actually feels comfortable talking to, and was actually supposed to tell Partridge about the rebellion—but then Partridge escaped the Dome a bit early, which sort of ruined the plan.
That's all we really get out of Glassings; he works for the outside of the Dome, even though he's a teacher inside of the Dome; he's an inside man.