Why do bees swarm?

Today a swarm of bees came into our school and are nesting. I've done some research on Why do bees swarm? Well what happens is the scout bees go and try and find a new place to nest in. These Bees have nested on a little not quite yet grown rose bush so all of the Worker bees, Queen bee, Scout bees and another thing I have Found out is that you can walk though a swarm of bees without getting stung unless provoked.