Mont. Code Ann. § 20-5-404

The governing authority of a school other than a postsecondary school may allow the commencement of attendance in school by a person who has not been immunized against each disease listed in 20-5-403 if that person has received one or more doses of the vaccine for each disease listed in 20-5-403, except that Haemophilus influenza type "b" vaccine is required only for children under 5 years of age.