I suspect the school asked because their computer system where they keep pupil contact details etc, doesn't have the option for Ms, or it requires a bespoke entering of the info, which they hoped to avoid.
The fact you replied but then received correspondence to Mrs, suggests your reply wasn't logged.
TBH, I don't imagine that the school was trying to make a point,mreligious, moral or otherwise about your marital status....it was purely an admin issue, down to computer systems.
I wouldn't be huffy about it in a response, just let them know you replied to their query, but the reply doesn't seem to have been logged. End of!
Incidentally, i have noticed the use of titles varies by institution depending on the 'type' of user they expect to have.
So, NHS, when I was having my DD - very careful not to call me Mrs, or refer to DH as DH, even when I did. When Form filling after birth about the Father, asked if I was legally married to the Father who I called DH - that made me laugh! I said 'yes I am married' and they asked again just to confirm, as if they couldn't believe anyone actually is married.
And then thinking of my DDs school (which in independent) I have noticed that they always, always refer to everyone as Mrs......They never refer to partner, but if asking about the other half coming to events, always refer to husband or wife. And there, I've never heard anyone refer to their 'partner' actually, now I come to think of it. Their are divorced parents,mso single parents, but as far as I've noticed (and I've not really thought about this until this thread) all the single parents were once married to the father of their child. Some of the divorced ones do now have new .....well, partners .....is the right word isn't it. But I just haven't heard anyone refer to them as such.
Are more of the middle and prosperous classes married? Are those women more likely to take their husbands name? Just got me wondering.