I'm actually going to agree with the people saying 'masks aren't going to do much'. I think they're right. Even the Telegraph was reporting yesterday that the government have fucked up, gambled and lost, and that we are now headed for a worrying period of chaos. The Telegraph author was free to report this (the Telegraph being famously anti-restriction so generally presenting covid data in its most positive light) as it is now
too late to introduce any restrictions that will prevent it from happening.
It's too late for the government to be doing anything in schools that will significantly ameliorate the impact of omicron. Staff absences will happen. Schools will be in dire straits and start falling over.
The mask guidance is accompanied by various other panicky guidance about remote teaching, combining classes and so on. They've already put out their emergency call for anyone with a teaching qualification to return to the classroom.
They are only introducing masks in classrooms because they know other, much worse measures (kids being sent home, maybe exams cancelled) will be necessary due to their fuck-up.
They don't want to be in a situation where kids are at home with no teacher having to answer questions about why they didn't implement 'simple' measures like masks in classrooms to try to prevent it happening.