How much time did you spend researching your list of names op? I've just spent an interesting 20 minutes...
lewesmum is correct - Will Seaward and Paul Sackey both attended John Fisher School during its "selective period". Will Seaward's parents are indeed barristers. Paul Sackey's cousin was murdered in London a few years ago, but Paul's own father was ....wait for it...a bank manager (and not a Ghanaian hospital porter one might imagine). Sackey says education mattered to his father and he was sent to a very academic, rugby playing school because his father considered rugby to be more academic than football. Mr Sackey senior obviously passed the John Fisher parent interview with flying colours.
Johnny Lee Miller's parents were both theatre actors and his grandfather,
Bernard Lee, played M in the first 11 Bond movies - hardly surprising he became an actor then. Likewise, Rafe Spall. In an interview, Rafe Spall said, "Who wouldn't want to see Timothy Spall's son coming to an audition? "It opens doors. Children of famous actors who profess anything else are naïve."
Euan Blair is a ridiculous example for obvious reasons. Hayley Atwell's mother is a motivational speaker - any connection? Oritse Williams went to Cardinal Vaughn School (similar to, but not the London Oratory); he started playing the trumpet in junior school. I couldn't be bothered to research Safety Fire.
Presumably, the sixth form you describe has a large proportion of students from Black African or Caribbean heritage, therefore you don't want a long list of successful white celebrities. In any case, many of the suggestions made have a similar background as your original list, or at least don't identify with your audience: Stella McCartney, celebrity father, Ed Miliband, son of a Politics Professor, JK Rowling, born in a Cotswold village, Keira Knightley, actor's daughter who had years of costly, private tuition for dyslexia, Jo (Josephine) Brand, daughter of a structural engineer and grammar school educated. Even Daniel Craig attended a well known grammar school at sixth form for a brief while, Bradley Wiggin's estranged father was a professional cyclist himself and Kate Moss was discovered at JFK Airport returning from a family holiday, not walking down Croydon High Street.
Oh well, perhaps motivational speaking is all hot air and lies anyway.