There are a few teachers’ education textbooks that put teacher free speech rights in an objective perspective. One is “Public School Law: Teachers’ and Students’ Rights,” Allyn and Bacon, 1998. with a chapter on free speech for teachers. The authors are Martha M. McCarthy,
Even when speech serves to highlight a legitimate public issue, school boards may restrict speech that threatens to disrupt or jepoardize the security of a school environment. Sometimes the manner which the speech is uncovered (such as whether the teacher draws attention to it at school) is relevant. In some cases, courts held that personal speech could be proscribed even when it did not jeopardize the school environment, so modern personal speech that has obvious political implications can represent a challenging gray area.
Related blog entry: "Is there a de facto don't ask don't tell policy for teachers" Nov 2006, here.