If this is just a standard classroom, you should be able to pick everything up using just one omnidirectional microphone. You gotta figure, if the student in the back of the class can hear the teacher, then a microphone will too, so there's really no need for multiple mics. An omnidirectional mic picks up sound from all around it. So, place one in the middle of the classroom and you shouldn't have any problems

The only reasons you would want some sort of close up micing would be if there is a problem with background noise of if you need a really clear recording of everything.

The single omnidirectional microphone is an obvious choice along with the lapel microphone for the teacher. However, one of the acoustic traits of a square classroom with an omnidirectional microphone is that of that horrible Tubby sounding effect, standing wave problem.

One of the techniques that I used many years ago was when I was working for NBC television in Washington DC, I had to Mike one of the large rooms in the Capitol. In the past they used to just throw out a couple of omnidirectional microphones which gave you that lovely awful sounding Tubby room sound. I had recently heard and seen the then new Crown line of PZM (pressures zone microphones). I was very impressed with them and purchased a single one for myself. NBC had none but I created 4 of them for this particular job, utilizing Sennheiser MKE2 lavalier microphones that I carefully "gaff" taped to the walls, separating the capsules with just a couple of sheets of paper in thickness, facing the wall. The increased clarity was quite remarkable! The managers were amazed. Unfortunately these kind of microphones have seemed to fall out of favor with people (I still have 6 of them that I use from time to time) and in their place "boundary microphones" were born, which I don't care for as much. Now Crown, I believe still manufactures what they referred to as a "PCC" microphone which is a more directional version of the hemispherical PZM?

Now of course background noise such as HVAC can be a problem but if you use software like Adobe Audition or some of the others that feature a noise reduction algorithm, it's quite effective at removing the background noise. Remember however you will need a clean spot of just noise, without children, to sample for the algorithm to recognize and remove, the noise, from the rest of the presentation.

5.1? Go sit in the corner.
Ms. Remy Ann David

Thanks for the posts. Very helpful.

Happy to read that one only mic can be enough for the task.
But btw, if it proves to be not enough due to outside noise, is it common to have several omnidirectional mics positioned all over the recording space?
Or perhaps several directional mics targetting only a specific area, and thus covering the entire room?

Tbh i was (still am) that the typical background noise of a school would create some difficulty to catch the sounds, but guess that good quality microphones can help significantly.

Regarding the problems of a square (well, rectangular, and usually with a nearby atrium, so not even symmetric), could be beneficial to place the mic closer to one of the sides of the room, instead of near centre?
(maybe getting rid of the abovementioned problems?)

If, as you state, the quality does not have to be wonderful, just useable, you are making too much of this. An old audio cassette machine with it's built-in "all over" mic and horrible ALC(Automatic level control), sitting on the teachers desk, would "work" fine. Today? Give the teacher a lapel(Audio Technica's cheapest - around $200 "kit with everything", is fine.), aim almost any mic out into the classroom(I would use a trusty SM57 - $100, as that's what I have) and all will be well. Perfect? No. So? You don't think the mic is picking up the kids' question? Tell the teacher to tell the kid to "SPEAK UP!" Done. If you like, you can get two(2) AT wireless mics kits, "tape the second" to a mic stand, using another lavelier or the "optional cable" to use ANY other dynamic, like an SM57, in wireless fashion, so you can move it around easily for best placement without kids tripping over mic cords. A sand-bag around the base would still be a good idea - kids are careless......

Worst case scenario: You, at some point, add another mic to "catch a classroom corner" or you try some other type of mic to see if it's "better". Worry about that when it happens.

Actually, if you set some of this stuff up and try it, during non-critical time, you won't have to guess or worry.

Hint: the fewer mics the better. Not as fun to run, but MUCH better, overall.

While you're setting up and adjusting - no one in the classroom - just "general" room noise with AC running, whatever will be happening, room-noise-wise, when class is in session, would be a good time to record your "noise reduction snippet" for later. Even if you are not, now, sure what this is, just record/save a minute of "bare room", "normal level", so you have it later when you will know what it's for......

TG