The transition from primary school to high school is very difficult for a lot of children at 11 and there's very little preparation for it. Somehow they're expected to jump from a nurturing primary school environment - which is often like a second home to many children, and spending the day in their own classroom with their own teacher and all their friends, then all of a sudden to a regimental high school system of registration, then having to go to different classes at various points in the day, with little more than a couple of minutes to get from one class to another and being penalised if they're "late" despite the fact the next class may be at the opposite end of a huge building than their previous one.
They have to get used to not just one new teacher, but several. Uniform rules are generally stricter, blazers aren't allowed to be removed no matter how hot it is, and in early September when school starts it usually is still quite hot.
Teachers in high school generally aren't as kind as primary school ones, if a child is upset or distressed they're expected to just "get on with it" they aren't comforted and reassured as they would be in primary.
P.E. lessons are more regimental, and you're expected to shower afterwards which is distressing particularly for many girls at this stage, as they are going through many changes and might not feel comfortable with this.
You're penalised for not having the right equipment on the right days, which isn't such a big deal in primary schools.
They're only 11. They've come straight from the school they've attended since they were 4 years old, and all of a sudden they're sharing a building for more than 6 hours a day with 15 and 16 year olds. And high school children can be brutal.
I know Year 6 is meant to be the "preparation year" but I don't think it's enough. They're still very much in a primary school environment. And outside of maybe one or two induction days, they don't have much help with the transition at all. I think this is one thing our cousins across the pond get right in terms of the education system, by having middle schools (previously called junior high schools) as a kind of in-between stage to help ease them in. I really think this kind of system would be beneficial here.