How to mike students and eliminate background noise?

Some of the teachers at my school are working on National Board Certification and need to video small groups of kids working together. Our PZM mic picked up not only the voices of the four kids in each group but also every cough and comment from all over the room. What kind of mic would get just the four kids in each group and not every sound in the room?
Reply: Try a cardioid mic on a desk stand. A microphone with a cardioid pickup pattern rejects sound from the rear, partly rejects sound from the sides, and has a wide area of pickup in front. Two examples of cardioid microphones are the Crown CM-200A and CM-700. Compared to a PZM on the table, a mic on a desk stand will be a little closer to the students, which also helps isolation.