I voted YANBU, but I did want to address this point:
I would have thought it'd be easier to socially distance year 10/12 than a bunch of primary school children.
-There are studies which suggest under 10s have a lower rate of transmission than other groups, and are at lower risk of complications of coronavirus (There are also studies that don't suggest this). So it's perhaps not just about social distancing, but also the risk of transmission too.
-Many Y10/12 students use public transport to get to school- increasing the risk to the wider population.
-If they walk to/from school, they are likely to do this in groups and possibly visit each other's houses.
-For Y12 especially, there's no point in going back to school if they can't be with their subject teachers. This means changing groups and interacting with 3-4 different adults at least. This is perhaps more risky than having Y6 children in in groups of 10 staying with one teacher all day.
-For Y10 students to access options subjects, the same applies, but with higher numbers of students. They could come in and just do core subjects though.
-If they're in school, it's likely they won't follow social distancing rules when they're not in school.
-Some secondary schools have really limited handwashing/toilet facilities.
-It's a larger number of students coming into school in total, and there may be pinchpoints such as exiting/leaving the school where it would be hard to socially distance 200-300+ people.
-Most Y12s have free periods- it would be hard to enforce social distancing during these (and there may be a lack of space), and if they can't get home easily it's hard to make them leave the school during this time period.
-Because the numbers are higher, you'd have more adults in school potentially interacting.
Obviously YR/1 won't social distance at all, but I don't think it's clear cut to say that Y10/12 can definitely do it better than Y6. Most Y6s can follow instructions as well as a 14/15yo (and they may be more compliant) and it's probably easier to facilitate say 30-60 kids socially distancing than 240+