In the case to which you refer, the local authority was held to be liable for an accident to a boy who was run over by a bus outside the school. But, as with most court cases, it would be unwise to jump to general conclusions, as each case turns upon the particular facts.
In this instance, there was a history of difficulty and of numerous warnings given to the local authority by both teachers and bus operators. It is also worth noting that it was the authority which was held to have failed in its duty, not the school or individual teachers.
Paragraph 43.7 of the School Teachers' Pay and Conditions document states that teachers are responsible for: "maintaining good order and discipline among the pupils and safeguarding their health and safety both when they are authorised to be on the school premises and when they are engaged in authorised school activities elsewhere".
It would seem unlikely that waiting for a bus outside the gates would be construed as an authorised school activity, even though, as in the case quoted, the authority was held responsible for the arrangements for the use of a bus which they had hired a contractor to provide. A teacher might decline to accept responsibility for such supervision, on the grounds that it was unreasonable.
There is nothing to prevent teachers volunteering for such supervision and many do so, because they see the need to ensure that pupils are safe. In doing so, they are acting in the course of their employment and the school or the authroity, as employer, has vicarious liability, affording them exactly the same protection as they enjoy when working in a classroom or elsewhere.
The same applies in the case of a teacher's common law duty. A teacher, not engaged in supervision, seeing mayhem among pupils outside the school premises, cannot disregard it on the grounds that it is outside contractual duties: he or she has a common law duty to intervene and the legal protection would be the same as for volunteers.
Wherever possible, pupils should be required to wait on school premises until buses have arrived and teachers may be directed to supervise them. If that is not feasible, while direct supervision may not be acceptable, teaches who volunteer to supervise do enjoy appropriate legal protection.