Rumpole I'm a governor of a primary school in Wandsworth - every year we have Key Stage results broken down by a number of factors - free school meals, ethnicity, sex, etc. The school is at least 90% non-white.
Every single set of exam results show the same thing - minority ethnicity pupils outperform white pupils. Even those that spoke no English (AT ALL) when they arrived at the school aged 5,6,7 whatever have, on the whole, a MUCH better grasp of the foundations of English grammar, etc, when they reach 11 and leave for secondary school. Especially written English. The reason the Head Teacher assigns is that those children with little English feel pressured to fit in, and so work harder and progress at a much quicker rate.
The incidence of exclusion-panel-worthy behaviour (kids doing stuff bad enough to be expelled or suspended, in old terms) is higher amongst white students.
Now, I admit this is just one example, and anecdotal at best, but my experiences have greatly defied my own previous assumptions.
I find that staggering tbh, that those starting with no English do better. What kind of exams are they, dot-to-dot?
Nonetheless, that's much better evidence than anything I have so can't disagree, and it would also defy my previous assumptions like yours. Agree with Melon about the chavs also, a scar on civilised life.