The iPad is the most popular end user device in the K-12 classroom today.
Today's de-facto standard for K-12 deployments is 1 AP per classroom.
It's typical to see ~30 iPads per classroom most anywhere in the world. Less in some places, more in others.
iPads are "single spatial stream capable (1SS)" devices with dual-band, 20MHz channel width capability.
A single iPad can process ~37Mbps of downlink TCP unicast throughput on a channel while operating on battery power.
Wi-Fi is a shared medium, and iPads will share the ~37Mbps downlink throughput capability per channel.
A dual-radio 11n AP can process an aggregate of ~60Mbps downlink TCP unicast throughput for a typical classroom of iPads operating on battery power, due to airtime contention. An average of ~30Mbps per channel is typical, but may vary between 2.4GHz and 5GHz due to interference, noise floor, SNR, and other RF factors.
iPads are designed for excellent battery life, which is why they have 1SS 802.11n.
Bi-directional (2.4G -> 5G and 5G -> 2.4G) band steering is very important for maximizing the amount of throughput available to each iPad across an AP.
Load balancing iPads across APs assures that iPads will always have the maximum amount of available airtime and throughput, so load balancing controls should be available and dynamic.
Features such as Voice Enterprise plays an important role in load balancing and standardized fast/secure roaming, though both the infrastructure and iPad must support it. This is shortly forthcoming.
Using a high-end AP with a deployment scenario of "one AP per multiple classrooms" offers significantly less throughput to each iPad than a one-AP-per-classroom because the throughput limitation in this case is spectrum-based due to the iPad's 1SS capability.
Band steering all iPads to 5GHz with a 40MHz channel width won't help increase throughput because the iPad can only use the primary 20MHz half of the 40MHz channel, and all iPads would then share the same channel and airtime contention.
If a mixed environment of 1SS (e.g. iPhone, iPad, iPod), 2SS (e.g. MacBook Air), and 3SS (e.g. MacBook Pro) devices are deployed together, the ratio of 1SS to 2SS/3SS devices within the classroom should be considered before choosing the most appropriate AP (e.g. a higher-end AP may increase throughput for 2SS and 3SS devices and improve network performance).
It's a given that in homogeneous-1SS-client-device networks, 2SS APs are good enough.
The application layer should be taken into consideration in a network design because not every classroom will run the same applications at the same time (e.g. video, eBooks, browsing, etc.), and this not every AP will be under the same load.
With 1 AP per classroom, all iPads will be guaranteed a high signal strength and will connect at the highest data rates that they are capable of achieving.
Smart antennas cannot improve the throughput that iPads or APs will achieve when they are already connected at the highest data rate with minimal interference.
Smart antennas can cause unnecessary interference around the classroom by focusing and transmitting too much energy toward and past an iPad, into an adjacent or neighboring classroom.