Novel to the student up to a point—math students are taught a variety of proof techniques in each of their subjects, theorems that students are required to prove generally fall to very similar techniques.
I have a BS in Mathematics from a mediocre school (University of Arizona) and exams were always "prove this fact (which you have never heard of before)", never "what is the proof of XYZ theorem (which we studied)"
there is so much room for questions, it's quicker to write the proof than to search an answer. And a readily found answer would, for undergrad, less likely be in proof form, or too distinct for specific topics later on. And the instructor might become suspicious and interogate you, in which case, if the material helped you to understand, that's the point.
