Children leaving France, including for school trips, need a free document called an autorisation de sortie du territoire (AST) if they are not accompanied by a parent or guardian.
One of their parents should complete a form that can be downloaded to fill in (or to complete online, then print out) here.
The form requires key identity and contact details for both the parent and child, such as place and date of birth, and the parent’s passport details.
The latter includes date of issue and issuing authority.
This is marked on the main page of the passport and will typically be HMPO, for example, for a British person.
Validity dates need to be indicated, which should be from before the date of travel and up to one year.
The child needs to keep a copy of this when they travel, along with a photocopy of the parent’s passport and their own passport.