Section 34C. Whoever, being under twenty-one years of age and not accompanied by a parent or legal guardian, knowingly possesses, transports or carries on his person, any alcohol or alcoholic beverages, shall be punished by a fine of not more than fifty dollars for the first offense and not more than one hundred and fifty dollars for a second or subsequent offense; provided, however, that this section shall not apply to a person between the ages of eighteen and twenty-one who knowingly possesses, transports or carries on his person, alcohol or alcoholic beverages in the course of his employment. A police officer may arrest without a warrant any person who violates this section. A conviction of a violation of this section shall be reported forthwith to the registrar of motor vehicles by the court, and said registrar shall thereupon suspend for a period of ninety days the license of such person to operate a motor vehicle.