Not quite the same, but my daughter started yr7 at the local comp. We lived in what was classed as a deprived area, and the school has a very high PP. She has always been academically able, and actively tried to be the top of her class in primary.
By the end of year 7, her grades were slipping, and her attitude was changing. She was still easily being in all of the top sets in her class, but not striving so hard to get the top marks. She was questioning why she would even WANT to go to uni, and chats about careers were all ending up in jobs that would pay little more than minimum wage with little growth. Combine that with a bit of teasing about being a geek, and a best friend who was mouthy and attitudey to the teachers, we decided to take her out.
She started year 8 in another comp, but one in an almost-local town. She is in year 9, and its been a massive change and a hard year. She's gone from being top class to being middle/ average in everything, which she's REALLY struggled with. She's just finding her feet this year, with a couple of close friends, and joining some clubs.
It's been the best thing we could have done for her. She needed something to push for, to show her that it's not acceptable just to coast and accept her lot, that it should be the norm to try your hardest.
We travelled every day to get her to the school, cost lots extra for travel. We've actually just moved out of our owned home and into private rented, to give her the best chance. I would definitely be going private in your situation.