The first and foremost duty is probably the most basic -- handing out the examination itself. During this process it is important to make sure that every student receives a test and, if the different exams are to be given based on seating arrangement, that the tests arrive as intended. This is also the time to read out the test guidelines, hand out pencils and other necessary test-taking items, and remind students to use the restroom and get a drink of water before the test.
During the test itself, one of your key duties is to maintain the academic integrity of the examination. Make sure that students properly spaced out and not looking at others' exams. Monitor the room to ensure that students are not using cheat cheats or have test information written on any exposed paper or other items. Note any erratic or strange behavior. Do not allow students to check phones or PDAs during the exam.
You are responsible for ensuring a suitable test taking environment by limiting outside distractions. Ensure that the room is quiet and that no one, including yourself, is participating in audible or otherwise distracting behavior such as eating, smacking, tapping pencils, shuffling feet and other activities of the sort. If a situation arises where a student is distracting approach the student one-on-one so as to not distract the rest of the class as they take their examination.
Collect the tests and any other materials administered during the examination when the test is done. Make sure that you have the same number of tests that you handed out and organize them as the teacher or testing administration asks so there is not confusion.