The American Civil Liberties Union says the law is likely to prompt a lawsuit.
Republican Bryant signed Senate Bill 2633 on Thursday, and becomes law July 1.
It says all school districts must adopt a policy to allow a "limited public forum" at school events such as football games or morning announcements, to let students express religious beliefs. The policy must include a disclaimer that such student speech "does not reflect the endorsement, sponsorship, position or expression of the district."
Bear Atwood, legal director for ACLU of Mississippi, says the group will wait to see if there's proselytizing in schools before deciding whether to file a lawsuit.