AOG
You need not necessarily worry about the 5% of kids who aren't Muslim. In the UK now, halal meat suppliers are increasingly turning to a form of slaughter which is both humane, and meets halal requirements.
As long as the animal is stunned (electric bolt) so that they cannot be revived, the throat can then be slit complying with the strictures of halal.
So, as long as the well-being of the animals in question is not compromised, is there any reason why the kids can't eat halal meat? Does it taste any different?
I suppose the school may have taken this practical approach because to supply the 5% of kids with non-halal meat would mean using two different suppliers, which would reduce the cost benefits of buying in bulk from one supplier.