You've banged on about the first part before, but why is it relevant to a debate on school standards being affected by kids? The parents don't go to school. (And I'd question the quality of some European degrees, as someone who knows people who've studied in most countries on the continent, on a year abroad from the UK - some courses are a joke.)
With the second part - London schools perform better, but that doesn't mean migrants/children of migrants aren't negatively affecting the development of 'native' kids, unless you can evidence that claim? Immigrants/children of immigrants perform better as they're more hard working and so forth - why can't that explain the better performance of the schools? (So essentially the argument would be that the 'native' kids perform the same, or even worse, but the affect on the average of the hardworking 'immigrant' kids outweighs that and leads to an overall figure that looks very positive).
Overall. But not all of them. The position could arguably be further improved (a greater overall net contribution) if the net drains within that group were stopped from coming here.
Just for you
http://blogs.lse.ac.uk/europpblog/2013/05/27/immigrant-and-native-children-dutch-schools/
after controlling for differences within schools, that the educational achievement of native children is almost completely unaffected by the presence of immigrant children.