The CUK Campaigning team well worth getting in touch with, but start with the existing briefing
At said talk-through I would, at the very least, want to know by what powers the school would be prohibiting students cycling away from the school on a public road without a crash helmet. I suspect they're playing fast and loose with the law, which I would not think is setting a good example!
Our intention is purely to make sure our young people are cycling sensibly and safely
There is, as it happens, a nationally approved scheme from the DfT for this very thing. Bikeability (or Bikeability Scotland up here) is based on the DfT's own National Standards for Cycle Training, and you can find the various outcomes for the NSCT on the UK Government website https://www.gov.uk/government/publications/national-standard-for-cycle-training-outcomes
The pertinent ones are the 3 levels, and the only mention of crash helmets you'll find are in Level 1, specifically the first outcome where it notes:
Trainees wearing a helmet should understand how to fit and adjust it.
And that's it
. That is everything the National Standards outcomes have to say about helmets.
Police are usually keen on quoting the Highway Code which says you should wear a helmet. If you ask the DfT why they'll send you the 2002 Towner Report, which concludes it's been shown helmets help, but it's been pretty thoroughly hauled over the coals in having a very limited range of references limited to non-randomised case-control studies, many of the problems of which are highlighted in Goldacre and Spiegelhalter's BMJ editorial
and in Gill's discussion of helmets in Cycling and Children and Young People
Towner's report is given at least
as much weight as it deserves and the author still concludes (despite wearing a helmet himself) that the policy case (as opposed to personal choice) for even recommending them has not been properly made.
Plus the Goodwin Report I've already noted, where the specific best practice recommendation is exactly not
what the school are doing.
This is all good ammo, but try and get a good grasp of it before going in and remember that the best way of getting what you want is to make sure everyone thinks they're on the same side. You want what they want, the decisions they have made make sense in the light of the limited evidence they have, but you have better and more up to date evidence that can better inform the choice made.