Hi Matchpoint2
We've just "finshed" our French education - DD did Bac this year and is going to university in September - so have fairly fresh memories of college.
I think the negatives to college are the same as with primary to be honest - strict teaching, lack of imagination for many teachers, inflexile curriculum, lack of opportunities for extra-curricular activities, increased amount of homework and very long hours - however if you have been in the system so long then it is no surprise.
I think it's best to view college as a stepping zone - lycee (and 3eme) is where academically and socially things come into their own. My DD had a tough time in college but that was the age/teenage/hormone thing that you would find anywhere - 11-14 year old girls can be horrid and there are cliques and nastiness but I know that that can occur anywhere. Colleges are more aware of bullying and giving additional support but are perhaps (it depends where you are I suppose) a bit "behind" schools I have seen in UK (experience of friends etc...)
I think my advice would be to be prepared for a very tired DD for first term, let your DD embrace her "Englishness" - it has advantages at college (eg choosing another LV in 6eme and helping other students/staff), prepare her for the usual early teen dramas (would happen anywhere), get involved with the college early on (PTA, parent's evenings etc..), accept that you will be less informed that you were at primary school (I "lost" a lot of school-gate friends when DD went to college and consequentially my French took a dive!), don't let your DD fret too much about moyens and school reports, find something away from college for your DD to do (on Weds or Saturdays) to boost her self-esteem, circle of friends and interest in other things (my DD did music and art classes - she's not sporty).
Finally, try not to worry - she'll pick up on this! It is a lot easier for them than us and as she's been in the system then it will be a breeze. Despite all the negatives I've highlighted DD has no regrets about being educated in France and she has advantages and experiences now that have set her up for the future.
Best wishes