I guess it all depends on the teenagers' ages and whether or not they had friends over. They must have been unruly because the police don't go door to door trying to find underage drinkers...
This is the actual Montana state law..
16-6-305. Age limit for sale or provision of alcoholic beverages -- liability of provider.
(1) (a) Except in the case of an alcoholic beverage provided in a nonintoxicating quantity to a person under 21 years of age by his parent or guardian, physician or dentist for medicinal purposes, a licensed pharmacist upon the prescription of a physician, or an ordained minister or priest in connection with a religious observance, a person may not sell or otherwise provide an alcoholic beverage to a person under 21 years of age.
(b) A parent, guardian, or other person may not knowingly sell or otherwise provide an alcoholic beverage in an intoxicating quantity to a person under 21 years of age.
(c) For the purposes of this section, "intoxicating quantity" means a quantity of an alcoholic beverage that is sufficient to produce:
(i) a blood, breath, or urine alcohol concentration in excess of 0.05; or
(ii) substantial or visible mental or physical impairment.
(2) Any person is guilty of a misdemeanor who:
(a) invites a person under the age of 21 years into a public place where an alcoholic beverage is sold and treats, gives, or purchases an alcoholic beverage for the person;
(b) permits the person in a public place where an alcoholic beverage is sold to treat, give, or purchase alcoholic beverages for him; or
(c) holds out the person to be 21 years of age or older to the owner of the establishment or his or her employee or employees.
(3) It is unlawful for any person to fraudulently misrepresent his or her age to any dispenser of alcoholic beverages or to falsely procure any identification card or to alter any of the statements contained in any identification card.
(4) A person 21 years of age or older who violates the provisions of subsection
(1)(b) is, in addition to applicable criminal penalties, subject to civil liability for damages resulting from a tortious act committed by the person to whom the intoxicating substance was sold or provided if the act is judicially determined to be the result of the intoxicated condition created by the violation. (See compiler's comments for contingent termination of certain text.)
History: (1)En. Sec. 56, Ch. 105, L. 1933; re-en. Sec. 2815.115, R.C.M. 1935; amd. Sec. 1, Ch. 240, L. 1971; amd. Sec. 2, Ch. 94, L. 1973; Sec. 4-161, R.C.M. 1947; amd. and redes. by Sec. 24, Ch. 387, L. 1975; Sec. , R.C.M. 1947; (2)En. Sec. 38, Ch. 84, L. 1937; amd. Sec. 2, Ch. 226, L. 1947; amd. Sec. 1, Ch. 161, L. 1951; amd. Sec. 5, Ch. 240, L. 1971; amd. Sec. 6, Ch. 94, L. 1973; Sec. 4-439, R.C.M. 1947; amd. and redes. by Sec. 104, Ch. 387, L. 1975; Sec. , R.C.M. 1947; R.C.M. 1947, , (part); amd. Sec. 1, Ref. 74, app. Nov. 7, 1978; (3)En. Sec. 1, Ch. 26, L. 1979; amd. Sec. 4, Ch. 186, L. 1979; amd. Sec. 35, Ch. 68, L. 1987; amd. Sec. 2, Ch. 217, L. 1987; amd. Sec. 1, Ch. 448, L. 1989.