I think you've made the right decision, especially if you live in mainland Europe already. My impression is that continental European teenagers are often socially, culturally and mentally much better equipped to deal with exchange programmes than Brits at around 15.
The girl DD1 did her exchange with (who stayed with us for 3 months) was doing Abibac, so DD1 went into her Abibac class at lycée. We're in Germany, but the DC attend German-English schools, so DD1 is also now trilingual, which will be brilliant for her career (and for communicating around Europe).
One real advantage for us with the programme was that the teenagers themselves were responsible for finding their own exchange partners, via the programme website or wherever else, and we had a LOT of e-mail and Skype contact with the other family before the exchange went ahead. So we weren't 'matched' passively, we did the matching ourselves.
"I was a shy 13 matched with a 17 year old boy; one of DH friends lived in a small council house in Stockton-On-Tees and found himself matched with a family who lived in a French Chateau." Yes, obviously that kind of thing is silly and doomed to failure. The more contact the families have in advance, the better.
"If anything I am more concerned about the practical aspects of receiving an exchange student for so long than with my pfb heading off into the big wide world for 3 months." Yes, Mistigri, it's definitely something the whole family is affected by! My advice would be to definitely get all financial stuff worked out in advance about what is being paid for by each family. So each family took their guest child on one holiday during the three months, within Europe, and paid for all activities on that holiday. But DD1 wanted to carry on with her riding lessons in France, and it was clear that we should pay extra for those lessons ourselves.
DD2 is taking Spanish not French, and we'd love her to do an extended exchange in Year 9 (aged 15 or so). Do you know of a similar programme between Spain and Germany? I was looking yesterday and could only find programmes where you pay thousands to an organisation and your host family is paid, so it's not reciprocal. I think a reciprocal programme is actually better because it's a non-financial arrangement, the families are more motivated to look after each other's children.