Corrected entry: Whilst on the island, Jeff cannot drink because he is on antibiotics to treat a dental abscess. It is possible to get different antibiotics that you CAN drink with - why wasn't he prescribed them instead?
In the chapel, you can see the heads of those who tried to cross Harriet (the mad matron) preserved in formaldehyde. How did she get hold of so much of it? Wouldn't her husband or the headmaster be suspicious?