When Thursday has Nero at gunpoint in the club and tells him to drop his weapon, Nero states that the weapon is empty and demonstrates this by pulling the trigger whereupon we hear a click. This is not possible since Nero was holding a semi-automatic pistol. If the pistol were empty, the slide would be retracted and it would not be possible to dry fire. The slide did not even retract after he dry fired.
The friendly teacher Alun Bodnar spells his forename in the Welsh manner, with a 'u', but still pronounces it in the English manner, as "Alan"; it should be pronounced "Alin".
When referring to the pupil Stanlow, who is being punished for refusing to fire a rifle by having to hold buckets up at arms' length, Mr Blackwell uses the expression "under heavy manners". The programme is set in 1968, but that expression didn't come into use until the state of emergency that was declared in Jamaica in 1976.
When the head is showing Morse around the school he casually tells him that in addition to his classroom duties he will also be a housemaster. Being a housemaster is a very senior role within a school - a position which one would need to apply for explicitly after gaining considerable experience. There's no way that a new recruit with practically no experience would be appointed as one.
He might have been attached to a house as a tutor.
He might have been attached to a house as a tutor.