I'm a teacher but have been off work sick so I've had loads of time to think (for a change!). Anyway, this is something that has fascinated me for ages and I'm sure there are posters here who know about this.
My school is fairly diverse. Most children with ESL parents come into school either with no English or basic words that have a distinct mother tongue accent. They lose this within several years usually, and by then they have a local accent.
I've noticed though some children do not adopt a local accent.
Irish Travellers: despite being born and raised here they nearly exclusively all have a very thick Irish accent, and this doesn't change over time. There are a few other children that have parents or a mother at least from different parts of the country, and they keep a very strong regional accent from them, which doesn't change much over time. This is an exception rather than a rule though.
Any language experts that can explain how this happens? I'm the embarrassing sort of person that goes somewhere for a few days and I pick up accents!