3 in 90
The '3 in 90' rules indeed refer to the 'carriage of passengers'!
A 'student' is not a passenger. Nor - it must be said - may passengers be carried unless it is a 'private flight' or is a 'public transport flight' when an AOC is required. Instructing is 'Ariel work' but is subject to exemptions from an AOC requiement for the giving of instruction to fellow club members. Any additional persons carried must be part of the lesson.