In Britain students of 17-18 will probably be working towards their A/S or A/2 level exams, and most of the sites for this age group reflect this. They're either specifically oriented to the exam or are in any case at an advanced level.
The site run by NelsonThorne, publishers of Au Point provides supplementary activities for the course which are useful for anyone working at this level. On the site you'll find grammar and vocabulary activties, and links to useful French websites related to topics covered in the course such as relationships, education, health and so on. Four stars.
The Channel Four website offers Homework High, a service where you can ask questions about French and get an answer from a teacher. They also provide some interactive activities, at four different levels, intended to accompany the TV course Extra. It's not as useful as it might be, but has a few things worth looking at. Two stars.