I was at school (I was a senior at the time) and they came on the intercom and told us about it. From that point on that day, every TV in the building was tuned to CNN.
A friend of mine, who went to a different high school, said that they tried to hide it from the students. They told the teachers to not say anything, and they wouldn't allow anyone to turn on a TV or radio. A few people called their parents/friends during lunch and found out what was happening. I understand not telling, say, grade school-aged children about something like this, because it's a job better left to parents. But high schoolers? That was extremely irresponsible on the school's part, in my opinion.