Bookworm. Back to back students are not passengers, they are students. It is permitted for an extra student to sit in the back for the purpose of flight training.
If it is acceptable to carry passengers on aerial work then why does any GA pleasure flight operator bother with an AOC.
It is also illegal to carry passengers on air displays, display practice, glider towing etc etc.
If you look at the definition of Passenger the CAA are quite clear on this.
The rule is there to protect the public, someone just going "for a ride" and putting themselves at a higher risk than on a conventional public transport flight.
I find it hard to believe that anyone can think that this is a legal and acceptable practice.