In Britain students of 17-18 will probably be working towards their A Level exams - either at level A2 or A/S, while in the States AP exams may be looming. A lot of sites specifically for this age group reflect this, with content which is specifically oriented towards the exams - though this doesn't mean they're not necessarily of interest to advanced learners in general. But if this is not what you're looking for, check out the links in the section on adult courses, where you'll find everything from beginners upwards.
Languages Online has specific material for both AS and A2, as well as a lot of general grammar and vocabulary exercises which would be useful revision.
If, on the other hand, you're working towards your AP exams, the EMC/Paradigm Publishing site for A Toda Vela may help. It contains exercises on grammar and vocabulary, as well as useful weblinks.
Practise your listening comprehension with these great videos from the Ashcombe School.
The Channel Four website offers Homework High, a service where you can ask questions about Spanish 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.