Thought I'd add to this, I was in private from year 3 up but had to go to state 6th form because of financial problems (23 now so not talking too many years ago) and it was fine, those taking a levels are the more academic ones, the chavs, dropouts etc tend to be on the 'easier' courses and have usually dropped out by Christmas.There was a good mix of people, a fair number of whome were also from private schools.
Having been brought up in private she probably has the same mantra as the rest of us private lot, A is what you want, B is OK and C is fail. The personal sense of pride is likely to prevail!
I had some terrible teachers and some fantastic ones, much like at my old private school and I think we benefitted from the increase in freedom and the more equal relationships we had with the teachers. For instance at my old school 'free periods' had to spent studying. When you hit university nobody makes you study so having more control over your time in 6th form is a good middle ground.
I also had more flexibility in a 6th form college to drop subjects and take on extras, there tends to be more scope for doing exactly what you want and not what fits in a smaller schools timetable.
At the end of the day I would say that after 16 her schooling is her choice, if she left school completely there is not much you could do so I would be happy that she wants to stay on, show interest, visit the local comp and other 6th form colleges around to see what she can do at them. You never know, she may change her mind and if not surely the most important part is that she is studying where she wants to be.
Oh and for the MSN? Google 'temptation blocker' and download it, you can then set it to block access to certain programmes (like MSN and the internet as a whole) for a chosen time. Like accidently setting it for 2 hours just when she asks to use the computer for some coursework then...!