Mo. Rev. Stat. § 171.021
Every school in this state which is supported in whole or in part by public moneys, during the hours while school is in session, shall display in some prominent place either upon the outside of the school building or upon a pole erected in the school yard the flag of the United States of America.
(L. 1963 p. 200 § 11-2)
(Source: RSMo 1959 § 163.030)