I had children there until recently. We are Christians. There were a range of beliefs - a few families actively involved in local churches, some of other religions/denominations, but I would say that perhaps half of children at St Michaels are not from Christian families.
As background, the St Michael’s school was set up by the church to educate poor children before education was compulsory in the UK. In 1880, education to the age of 10 became obligatory in the UK. Rather than just open lots of new schools, the government began to contribute to the existing schools operated by churches. In Highgate, the church owns the large and lovely site of St Michael’s, but most of the operating costs are paid by the local authority. Being a Church of England school means that the church and diocese appoint members of the governing board, and that the school has a religious character.
Assembly - the daily act of “worship” in UK schools is led by someone from a local church once a week, but on other days often had very little Christian content, according to my children. Some of the big events are Church orientated - all the children make Easter bonnets to parade from school to church for the Easter service, at the y6 leavers’ service the children are given a bible.
They follow the national curriculum - you can see more info on the website. This means that for science and history, the teaching is factual - creationism will not be taught in science. There is no national curriculum for RE - I think they follow the Diocese of London curriculum. This involves spending some time learning about Christianity, but also plenty studying other religions.
When you asked about the creation story, I guess the teachers thought this was a question about RE - there are two creation stories in Genesis, and so I imagine they teach both…
Personally, I would have liked more Christian content in subjects like RSE. They used a secular curriculum, but I would have liked one with more Christian values - if faith means anything it should permeate our relationships.
In terms of academics, they don’t prepare children for 11+ exams. Families use tutors, or parents tutor. However, the school attracts a bright bunch of children and teaches to a high level.