The Russian authorities did not plan to storm the school. There was a lot of chaos because the townspeople all had guns and wouldn't get out of the way.
People in the school think an explosion was set off by a terrorist by accident. When the kids ran, the terrorists shot at them. Then the Russians gave covering fire. They had no choice then.
Here is an article that says they did not plan to "storm" the school. A doctor named Roshal who has been involved in handling other terrorist attacks said nobody wanted to storm the school.
Everyone wanted to negotiate.
http://www.rambler.ru/db/news/msg.html?s=260005085&mid=5003793
It turns out that a planned attack would have been the best thing....at least...no worse a thing.
I read an interview with a child who was in the gymnasium. The terrorists had taped up bombs in various places, including in the basketball goals. The tape came loose from one and the bomb exploded. This apparently killed many hostages and collapsed a portion of the gymnasium roof.