In the school I used to work in, a cohort of 150 needed 15 invigilators to staff the core subject exams, accommodating all the special arrangements, readers, scribes, etc. In order to guarantee this, the invigilator pool was up to twice that. This was to give sufficient back-up to cover invigilators' personal availability (they are mainly employed as casual staff and have other work and commitments) and possible illness (reasonably likely given that many were 60+) and also to have enough staff in reserve in case of student illness requiring exams to be sat at home (two invigilators required if that happened). Student anxiety about exams in a normal year can easily affect students, especially those with pre-existing conditions, such that they cannot go in. Imagine the huge increase in that situation this year in view of the level of fear about the pandemic, especially if exams had to be held in unfamiliar places, even just different rooms, with inexperienced staff (and JCQ rules demand that everybody has had full invigilator training in the academic year the exams are held - you cannot just go and grab the nearest teacher to fill the gap).
Several days before exams were cancelled, year groups at schools I know of were being sent home because self-isolation and shielding guidelines were coming into effect such that the school no longer had enough staff to teach/supervise them effectively.
And then there's the impact of students suddenly being locked down at home with their families for an indefinite period of time. Learning over Zoom, even if the school has been able to facilitate the teaching, just isn't that easy if you don't have adequate tech, for starters. And the kids with half a dozen siblings all crammed into a house, day after day, parents there too, with no tech of their own and no peace and quiet to work? Not so easy to carry on.
I have thought a lot about what would have happened to me if this situation had arisen when I was in Yr 11 or Yr 13. With an unemployed parent with complex mental health issues, we would definitely have been in the group of students who didn't have the tech. His behaviour would have meant that there would have been no peace and quiet to study, either day or night. The continued forced proximity, running into weeks and then months, while he repeatedly attacked my mother (and late teens was the age when he deemed me fair game for a bit of that as well) would have broken all of us and it is highly likely that during the lockdown period he would have killed either my mother or himself.
Nobody would have come to our rescue because outwardly we were articulate, "posh" and middle class. My parents put up a jolly good front outside the house.
My situation and variants of it goes on, and on, in other homes for other students - very very many of them, as I found out when I started working in schools. As a young person I always thought we were alone in our situation but we really weren't.
I do appreciate that school itself is a frightening and hostile place for many students. But I also know that for many others it's the only safe place they have, and once they couldn't go there any more, taking exams might frankly have taken second place to just surviving.