Looking at the picture, I can see eight 'black' people – the faces of four in the foreground and four in the background. Then there's Mr Clegg to the right and in the far right-hand corner of the picture there appears to be another white guy looking down at the floor.
Your question is entitled, “Clegg lecturing at a school and....” and your immediate question is “Not one pupil is white...”
So without taking your statements out of context or out of order, you're effectively saying that, 'Mr Clegg is lecturing at a school where not one pupil is white' and you're basing that statement on a photograph in the Daily Mail that shows Mr Clegg (and another unknown white male) with a grand total of eight black pupils...
I think it's fair to say that your assumption could be unfounded as according to Wikipedia, whites make up over 64% of the population of Southwark (2008 estimates). Even if your statement turns out to be true, I fail to see it's relevance.
http://en.wikipedia.o..._Borough_of_Southwark