Because this is UK focused it assumes you'll know that FSM = Free School Meal which is a proxy for household poverty. People who've seen this kind of work in other fields will recognise such proxies and probably assumed that's what is going on, but just in case.

It's an academic paper, so if written correctly it does not make that assumption (about the domain-specific abbreviation at least). Instead, the abbreviation will be written out in full at the first use, and it is:

> Slightly more children and young people who didn’t receive free school meals (FSMs) told us they enjoyed reading compared with their peers who received FSMs

To reinforce, it doesn't mean Finite State Machine or Flying Spaghetti Monster