I didn't say the police could create an identifiable fingerprint from it.
I said the police could take a fingerprint they have found in RL, apply the algorithm and then use the school database to identify it. Thanks for sharing the figure of identifiable only to 1 in 7000, btw, Bound, I've been wondering about that.
Do I think the current UK government would collect up school databases in case they came in handy later? No. Do I think a rather different govt would be delighted come in and discover it had a reservoir of data about every schoolchild in the country. Yes, it's happened before.
If it's just data about current schoolchildren, the potential is very limited. But if the schools aren't religious about deleting data - or it's leaking in any significant quantity - it will be a reservoir of more of the population.
In this scenario, it's probably not important if the data isn't itself admissible in court, the intelligence is usually what they're after.
Re "from a school POV its laughable to pretend there are other motives."
It doesn't matter what the motives of the collector were; what matters is once data is there, it can be used for other purposes.
I don't honestly know if schools passing (eventually) the majority of the population through their biometric databases will lead to harm. If deletion is carried out rigorously maybe not. But deletion is an area data controllers tend to be weak on - as discovered by RedGreenRouge's DD above.
Btw several schools described above seem to be in breach of the Portsmouth guidelines linked by edam, and RGR's DD's school may well be in breach of the law.