Yes, schools and school libraries can install Internet filter programs to block access to web sites with obscenity, child pornography, or any other content that is “harmful to minors.” [1]

[1] A federal law called the Children’s Internet Protection Act makes Internet filters a condition for libraries and schools to receive federal funds and discounted telecommunications rates.  20 U.S.C. § 6777 (federal funds) and 447 U.S.C. § 254(h)(5)(discounted access rates).  The Supreme Court has held the condition doesn’t infringe any constitutional rights of adult library users.  United States v. American Library Ass’n, 539 U.S. 194 (2003).