If you're looking specifically for more imagery by me, I'd recommend the new audio for Concentration, Focus & Learning (including ADD, ADHD and LD) and the one for Self-Confidence/Peak Performance.
If you're willing to branch out to other practitioners, there's a CD set by Shirley McNeal that specifically targets test anxiety, called Test Taking Mastery. I think it's very good. You can find it on her web page here.
We carry other wonderful meditations that are relevant to concentration and performing well under pressure - by Martha Howard, Emmett Miller, David Illig... but I'd keep it simple and stick with those three, unless you're starting months in advance and would welcome more options, lest you get bored with just those. If you only have a couple of months to prepare, though, those three should do it.
Hope this helps, and best of luck to you!