I used to work in a very mixed demographic school near a hospital so we had Doctors and nurses children, quite a few engineering type families due to boat manufacturers and some growing up on farms and also covering one of the most disadvantaged areas in the UK.
Children tend to mix more with who they lived closest to and had gone to Primary school with at least in KS3.
As they move up the school sometimes these mix abit more and you hear them talking about meeting at Xs house as "it's massive".
From a first impressions view, you could tell more who had engaged parents which isn't necessarily determined by class, those children who had been read to, those who went on holiday, or visited London or went to the theatre or museums. We have plenty of free or very cheap venues around us so it wasn't a money issue, it just wasn't a priority for some families. We lived by the beach and many of our students had never been (and I mean I could see the sea from my classroom).
Parents have different values too, some latch key children with high achieving but often working late parents and those with more lax parents for other reasons would mix more as there was less supervision and not always for the better. Those parents who were more involved had their children in more clubs and activities and sometimes friendships from these carried over into school.
You can tell by the clothes and colour of the shirts, sometimes the richer more rural students are messier / look like they've been dragged through a hedge backwards as they've been up X hours already but whilst their shirts might be greyer they have good quality shoes on and fresh uniform when needed and were generally quietly confident, opposed to those whose shoes were literally falling apart, grey shirts, couldn't concentrate as they hadn't eaten properly, generally look pale and shallow and either big bravado characters or extremely meek. Generally the poorer families had more children so we had some families with 8+ children but no quiet place to study or expected to look after siblings. We had lots of families that had children with other disabilities that has a knock on effect of what the family can do and access etc.
But overall I dont think it's directly a class thing, it's a parent engagement and priorities thing.