I think that depends on the materials of the walls and the density requirements within classrooms. I also try to keep them "as far away as practical" as a rule, but in our classrooms, we haven't often found that to be very possible at a great distance. We have plenty of high-density classrooms and lecture halls with 2,3,4 APs all within 10-20 feet of each other. Most other classrooms are 1 AP per room, probably about 10-15 apart with walls in between. We've had it this way for about 3 years now and performance has been pretty solid. Our profiles keep them at 20 MHz channels, channels 149 and 153 are disabled to prevent interference with Airplay devices. In areas with drywall walls, we turn down transmit strength on 2.4ghz, but many of our buildings are much older and don't require that due to the contruction materials.
But, if it was new construction and I had the choice of placement, I'd expect omnidirectional APs to be placed in the center of each room. Having them hug the walls doesn't make sense to me.
