Not all of the boys were students and choristers at a cathedral choir school; only some were and they were being led by the "head boy".
There were NO adults present, and though it is true that many Brits said prayers prior to partaking a meal back then ( far less so now do that ), having the boys pray, in the story line, does less than nothing at all re the story!
Also, I doubt that you are personally familiar with such Brit schools. OTOH, I am and they were extremely strict, rigid, and harsh; ergo, it is really not all that "strange" that once set "free" of adult control, that those boys, in particular, went wild.
In other words, are you saying that even in the 1950s there were no real Christians among cathedral choristers in the UK and when they acted like Christians (in the presence of adults), it was only make-believe? Not even one boy on the island (chorister or non-chorister) said: “What you are trying to do is sinful and you may go to hell because of this.”
I must compare this with tragic circumstances in the Middle East where a few years ago a Christian boy was captured by ISIS. They wanted him to convert to Islam, but he chose martyrdom.
If this book is an accurate portrait of British private schools and society in the 1950s, it means that Western civilization was dying much earlier than we usually tend to think (and gay “marriage” and LGBT+ problems are just late symptoms of this dying process).