Just because you feel that you have a concept or subject well in hand does not mean that you can pass a test on it with flying colors. This is especially true in math and in tests where a broad level of subject matter is covered. And, there is one simple reason why.
Students are taught content and more content, but are not often taught to practice that content quickly. Over years of test taking and teaching subjects where the goal is to pass a test, I always have my students get a timer. A simple kitchen timer does the trick, and you can find them all over the place if you take a look.
I never start any student off with a timer because I don't think being timed while still learning a subject helps anything. (And, in fact, I think it hinders learning to be hassled by a clock when you don't have a subject mastered.) Once the student shows that they understand some portion of the subject, however, I bring in that timer. I ask them to practice what they now find "easy" with the clock going. This not only reinforces the skills that they have mastered, but reinforces the need for speed. In my ideal world we wouldn't need timed tests, but, hey!, that's not what most schools do.
Not only can a timer help you practice under "pressure", but it can also help you get your schedule in order. You will find that a lot of time management folks and other tutors completely endorse the use of timers because they work! We all seem to have over busy schedules, and one great and easy way of getting that schedule under control is to set time limits on tasks so that they don't eat up the time needed for another. I won't go into those details here, but I do want to emphasize that students should invest in a timer for test taking practice. After that, they can learn how use that same timer to get their life organized so that they can have more time doing stuff that isn't necessarily school related.