I would make schools much smaller and separate out years 7-9 from 10-13. Too many children get traumatised and overwhelmed by the sheer vastness and anonymity of secondary. Unnecessary.
I would have nationwide thorough training on bullying and how to tackle it from every POV - the victim, bully, parents, staff, so that it is not brushed under the carpet. I would do that thing a US teacher did where she got everyone to write down the names of two people they'd like to sit next to the following week, and then check which children weren't chosen by anyone. Those children would be supported subtly to develop social skills so there are no social outcasts in any class.
I'd ensure that physical fitness was prioritised and that pupils who are shit at team sports simply got to choose individual fitness programmes instead.
I'd teach life skills: relationships, personal finance, resilience, boundaries, time management, how to keep a home tidy, clean and well-organised, how to build a life worth having, how to make friends in a new place or if your old ones have drifted away, job interview skills, how to disagree with another person;s POV without a fight or sulk etc.
All food on site would be super healthy and nutritious. All schools would have playing fields, and would at least share a swimming pool that enabled every class to swim at least once a week.
All schools would offer triple science and relevant foreign languages like Egyptian Arabic and Mandarin as well as European languages.
All schools, as well as having A-teams for sport and arts would have all-welcome teams. One of the things I loved best about my shitty comp was that they let me into the school choir and orchestra. I was rubbish at music. But I loved it and have very happy memories of concerts.
I would get rid of any high-salaried middle management and focus on excellent teaching and support staff. Since it is a fantasy, I would give teachers a massive pay rise, hold onto experienced staff with excellent bonuses and quadruple the budget for each pupil.