Don't Cry with Your Mouth Full(1973)

Original title: Pleure pas la bouche pleine

Annie, a 16 years old girl, loves Frédéric who is away doing his national service. During her holidays in the country, she meets Alexandre. He's older, owns a car and she experiences the "first time" with him. Back during a leave, Frédéric finds his sweetheart in the arms of this clumsy oaf and eventually hits it off with him.