In order to receive Federal E-Rate discounts, public schools are required to have filtering mechanisms in place that meet the standards set by the Children's Internet Protection Act.
I've administered K12 networks with internet access for over 15 years (both pre-CIPA, and post CIPA)... I personally preferred not having to filter and teaching personal responsibility, especially with high school students. Usually a couple times a semester a student would make a bad choice, and would be made an example of.... which would usually keep the rest of the students on the straight and narrow.
But for now, CIPA is the law of the land, so if you want free choice and thinking on your school's internet connection, contact your senator and congressman, because local admins really have no choice in this matter.
Hackers of the world, unite!