What your principal is doing appears to be against the law and a violation of the constitution (First Amendment; separation of church and state). Probably the best way to address it if you are offended or concerned is to either speak with higher-ups in the administration (e.g. your superintendent or an assistant superintendent), who may be unaware of this behavior; or if that is unavailing, or you suffer any retaliation, to speak with an attorney about legal action. One possibility is to contact the ACLU--their purpose is to protect civil and constitutional rights.