Showing clips of PG certificate films - UK schools

[EssJay1]

It's recently been raised in my school that teachers can't use PG certificate film clips unless parental permission has been given. I've looked for guidance on the WWW, but can't find any - the BBFC just describes the conditions imposed on a film in terms of its content. Strangely, they are more explicit about 12As where the supervising adult can decide whether the film is suitable for the child.
Can anyone clarify or point me in a direction where I might find guidance. I fear it's one of those teacher myths that becomes self-perpetuating, but actually has no truth behind it.