The powder contained in fire extinguishers, while not recommended purposely breathing it in, is generally harmless in the long run. Due to being a fire suppressant chemical it obviously will have some effects, such as the burning sensation you experienced, but no harm should have been caused. Unless of course you are still having any types of breathing troubles, if so you may want to hit the emergency room.
The fire alarms were most likely automatically set off by the dust-like chemical coming from the extinguisher. Most modern day fire alarms check for a variety of things such as smoke, heat, carbon monoxide, and potentially dangerous levels of particles in the air which is probably what set this alarm off. No matter what when an alarm is sounded it transmits immediately to the first on-call fire station which will send over a few trucks to check out the situation. These first responders will either call a code 4, which means no further assistance is needed (you seeing the third fire truck called off) or call in additional units needed to control the emergency.
Obviously with the school evacuated and the cause completely controlable and putting no students in harm, no further fire units were needed and the situation was easily controlled.
