How old is your grandson? I presume primary school age? Can he manage his diabetes himself, or does he still need an adult to help him?
They are probably locking the meter away because they are frightened that it will get lost or fall into the wrong hands, however as you say that's not much use in an emergency. He should have a care plan set up explaining how his diabetes should be dealt with while in school, it should also say that his test meter and hypo treatments should be kept on the classroom so that they can be found quickly in an emergency. Schools these days have to have a member of staff willing to be trained to do whatever is necessary to help a child with a medical condition (although this does not necessarily have to be the class teacher).
You and his mum need to get together and decide how you would like his diabetes to be dealt with st school and then arrange a meeting with his teacher / head / deputy head or whoever is appropriate, and try to make it clear that locking away emergency medical equipment just isn't acceptable, and explain why. If the school has any concerns, listen to them and try to come to an agreement - although having said that, they need to understand that he MUST have all his equipment within easy reach and be allowed to use them whenever and wherever necessary, and absolutely cannot be expected to go wandering around the school looking for his kit if he might be hypo! He also should not be expected to leave the room or in fact move very far at all when dealing with any diabetes issues. If you don't have any luck getting the message across then maybe his diabetes nurse can help? Ours are really good at dealing with school staff who don't understand!
My daughter has always been allowed to keep her meters and hypo treatments in the classroom and use them whenever necessary, and nobody had ever had an issue with it, although when she was at primary school, I did used to have a meeting with every new teacher each year to make absolutely sure they understood. Now she's at secondary school it's in her care plan that she must be allowed to keep her bag with her at all times (or within very easy reach for lessons such as PE) and apart from a couple of teething troubles with certain teachers who insist that all bags are left outside the classroom, and didn't seem to realise that my daughter was the one they had all been told about who is allowed to break certain rules, it's all worked really well. (Said teething troubles were all sorted out very quickly I'm pleased to say)
Hope you can get a better system sorted with your son's school, good luck 🙂