Reply: The Crown Sound Grabber II is a good PZM microphone for your application. It's available for $69 from Sweetwater Sound, among other online stores. Here is the web link:
If you stand in the classroom where the microphone is located, the Sound Grabber II (or any microphone) will pick up roughly the same amount of background noise that you hear in the same position. So if your room sounds noisy to you where the mic is located, the recording will be noisy too.
Pro audio engineers put a lavalier (clip-on) mic on each person speaking, or use a pass-around handheld mic, to totally eliminate background noise. If your recordings must have no audible background noise or room acoustics, you would need to add an extension mic cable to the Sound Grabber II (or any microphone), and pass the mic to the student who wants to speak. That way, the mic will be a few inches from each person speaking, and so the mic will pick up little or no background noise. With any microphone, there will be some mic handling noise when the mic is passed to the student.
If you can live with a little background noise and room acoustics in the recording, try using one microphone in one of the locations in the drawing:
• One Sound Grabber duct-taped to the ceiling over the front row of students.
• On the floor, or on a table, as close to the students as possible.
• Remove the "paddle" or boundary from the Sound Grabber II, and mount the "nose" of the mic in an upper-front corner of the classroom (a wall-wall-ceiling intersection).