I don't think they need a release as there's no recognizable person in the shot. All their heads are covered.
Branding on backpacks and clothes would be more of an issue if they're really big, and you wouldn't be able to get a release for these.
It's always a bit of a gamble with this type of shot as some agencies will reject it and some will approve it depending on the reviewer's mood.
Get the shot anyway, you've got nothing to lose. Worst case scenario you can submit it as editorial if it's rejected.