Your regsitration will be simpler if you have students choose from a list of exams reather than letting them "fill in the blank" with the exams they would like to take.  Letting students give a free response is a recipe for disaster as students often times do not us the official course title.

It is important to list only the exams that you are willing to offer.  Students are much more inclined to register for the correct exam if you give them a list to choose from.  You can always include a note for them to come see someone if they would like to take an exam that is not listed.  This way you can make a decision on whether it is appropriate to give an exam for only one student.