Jeophal the Spry was a Wizard mentioned in Equal Rites as one of those unfortunate members of Faculty who teach students. Specifically, he taught Beginners' Dematerialization to second year students in Unseen University's gymnasium- a room designed for the practice of incredibly advanced magic and lined with lead and rowan wood. The goal was to prevent student wizards from seriously unbalancing the universe, but they often ended up unbalancing themselves, and were removed in bottles.
The classroom he typically used was empty when he taught classes in the gymnasium, and Simon and many other wizards commandeered it for a lecture by Simon. Esk, thinking the classroom to be empty, was sweeping up in there and inadvertantly listened to Simon's lecture.