The first appearance is in the first book, at night, where the department teachers are seated along with Dr Harold Bloor. From a high leveled height, the staff can monitor what happens around the Dining Hall. The High Table is on a raised platform at the end of the long room, horizontal to the other tables.
Before Bloor's Academy became a school, the Dining Hall was a dungeon where the Red King's children held their prisoners. The High Table came from the idea where someone "high" would have the opportunity to be seen.