"In secondary schools you at least have the option of standing away from students, and keeping a distance - which is more than can be said for nursery and primary settings. There is not the need to hold, pick up or care for children at very close range."
In some classrooms there physically isn't enough classroom space to be 2m from the front row. Even if you squash yourself up against the front wall. Class sizes have steadily increased due to budget cuts over the years in my experience.
Seeing as transmission is thought to be airborne - I am therefore in a small room, with few windows that open (especially if not in a ground floor room) with 32 students facing me directly and sitting shoulder to shoulder with each other. I can stay at the front, but I am not always 2m away and I have 30 people breathing towards me in a fairly small, poorly ventilated room.
"I don't think masks do very much in reality" well I think I'll listen to the scientific experts on that one, who seem to think they do help when inside in poorly ventilated spaces. Why on earth are they asking everyone to wear them in shops and public transport etc. and in many places around the world if they don't help?
Re running out of staff - that is definitely going to happen in some schools. Touch wood we haven't got many staff out, but a nearby school only needs one more staff member to be off and they will have to start shutting year groups as they won't have enough members of staff to safely run the school. The problem is usually teachers often go in when unwell - we aren't allowed to do that now. If someone has to self-isolate for 2 weeks you can teach remotely by being streamed in but you still need a physical person in the classroom. There is no money for supply Of course staffing is going to be a big issue compared to normal times.