My two cents:
It would be one thing if it were a bunch of 17/18 year olds who were either eligible to vote now or would be eligible at the next election; if the information was presented in a factual, unbiased way, which makes it educational and un-partisan; and if there was no follow up of "I vote this way, you should to".
But it didn't happen that way. It was to a bunch of primary aged children (in this country, year 8 is twelve and thirteen year olds, and it's the last year of primary school), and I know first hand how those children often hear what they want to hear and can and do present a very skewed and less than factual picture of "what happened at school today" to their parents. And if you get the parents offside, being a teacher is very hard indeed.
As a teacher, no, I don't think you CAN be seen to have very firm opinions on matters such as politics. In my classroom I have a very strong opinion about bullying (I'm anti), and a very strong opinion about respect (firmly pro). I'm happy for the kids to know about. That's Ms. inthesound during school hours. Faith outside of school hours can be as opinionated and political as I want, can go protest and burn various items of clothing and picket and so on and so forth... because that's my time as an adult person. When I'm at school, I'm a teacher first, and learning is priority one, and politics have to take a back seat.
I am actually a very politically aware person. I have hugely strong opinions about social justice matters. In fact, there are certain policies at my school I'm incredibly uncomfortable with (one in particular, not because it's that awful, it's just archaic and against my feminist sensibilities lol) but none of that matters when I am Ms. inthesound.
So yes. If it had been teenagers, with which you can have actual reasoned and nuanced conversation, I wouldn't be AS concerned. But with primary school kids, who so often seen in black and white, don't understand nuance, and report things in such a skewed manner, I'd be horrified to speak to them about my personal politics. I don't think you were wrong to say something. I agree with a PP I probably would have caught her in the hall later and had a quiet word, rather than do it in front of everyone, but I still would have said something. The kids are too young to understand what she was trying to do.