It's new that all schools have to but yes, most secondaries were already doing this.
I don't know how many other schools with 11+ year olds were though. I'm in a prep and our Year 7 and 8 were asked to wear them on Monday for the first time in anticipation of this becoming the rule on Thursday. Might be new for middle schools too.
To be honest, I can't see that it will make much difference. It means 1/3 of our children will be wearing masks for maybe 20-30 minutes of their day. That's not much mitigation. More of a PR exercise, I'd say.