I had not looked too much at WolframAlpha yet, but it looks very handy. Lots of Mathematica can be done before hitting the time limit, and many nice added features are there.
For example, the student wishing to test the correctedness of her solution to some random calculus problem can check it herself by using the “show steps” button, which provides explanations of chain rules involved, and so on. Nice. (But then, another good reason to make sure students switch off their phones during exams…)