We think they might have something to do with the ventilation system that Rowland Hill introduced to the building when he was headmaster of Bruce Castle School from 1827-1839.
In the north view of the building below, circled in red, you can see two odd-looking and large ventilation shafts emerging from the roofline - perhaps one of them is the very same that we have recently uncovered during our building works! Rowland Hill was a keen experimenter and one of his interests was in getting fresh air circulating in the building for the young pupils in his care at Bruce Castle School. It's wonderful to be able to get a glimpse of one such experiment, previously hidden for so many years behind ceilings and floorboards...