I've seen this solved here in slovenia in (at least) one school, where they have a "classroom phone" (a dumbfone, calls only). All the parents know the number,

I've seen this solved here in slovenia in (at least) one school, where they have a "classroom phone" (a dumbfone, calls only). All the parents know the number, all the kids can use it in an emergency. They are usually not used in class, but on a field trip (especially multi-day one), the kids use it to call their parents and vice-versa.