The thing that is very important to realize here is that this case does not tell states or school districts what they must do. Instead, it tells them what they may do. In this case, the Supreme Court told the Hazelwood school district (and others) that they were allowed to have their principal censor a part of the student newspaper that was, in his opinion, contrary to the educational goals of the school district.
However, this is not a requirement. The Supreme Court did not tell all school districts that they had to have their principals censor school papers. This was a case that was about saying when schools could abridge their students' First Amendment rights, not one that was mandating when schools have to censor their newspapers.
So the answer to your question is that states do not have to "opt out." There is no requirement imposed by Hazelwood. This case merely tells what schools may do and it is up to the school district or state to decide if it will take advantage of this ability.