I don't believe OP said she was shaking in fear or hiding behind the sofa or anything like that? Maybe I missed that bit...
It doesn't matter that some people wouldn't care, or think it's lovely, or that a child couldn't wait until the next class, or that the OP is "batshit", private data like an address isn't meant to be used like that, no matter how "kind" the intentions. Some of the principles of GDPR include lawfulness, fairness, transparency and purpose limitation, e.g. must have a valid legal basis (e.g., consent, contractual necessity, or legitimate interest) to collect and process data. Data handlers must not use the data in a way that is overly detrimental, unexpected, or misleading to the individual and they must be completely clear, open, and honest with individuals about how their data is being used. Data must be collected for specified, explicit, and legitimate purposes. You cannot use, repurpose, or share the information in ways that are incompatible with initial, agreed-upon purposes.
I understand teachers feel attacked, but professional principles (and potentially GDPR law) can't be based on the fact that many mumsnet posters wouldn't mind. All the teacher had to do, if they really thought the child would be that upset, is spend 30 seconds ringing up OP to check this would be OK. I mean, as posters have pointed out, the OP's data (phone number) would have been instantly accessible.