A Missouri teachers’ union said Friday that it is challenging a new measure that restricts teachers’ use of social networking sites and their contact with students, saying it violates their constitutional rights. The Missouri State Teachers Association said it is seeking an injunction to block enforcement of part of a law that takes effect Aug. 28. The union and several public school teachers assert the law violates educators’ constitutional rights to free speech, association, and religion. Many teachers have complained the law will hurt their ability to keep in touch with students for classroom purposes, personal problems, or even emergencies.