If you've taken this kind of pill in this dosage for specific anxiety-provoking events with success, then yes, take a small dosage.
If you've taken it for general anxiety, don't take it before the exam. The changes in brain chemistry are not immediate.
We have an adult offspring with diagnosed social anxiety disorder. Her means of coping was to over-prepare for important things like presentations. She'd practice out loud in privacy, practice before a family member, practice before a video camera, and in general was way more prepared to do it, despite her anxiety, than any of her peers.
As always, I strongly recommend six months of cognitive behavioral therapy for anxiety, which teaches you ways to cope while desensitizing you to stressful situations.