Uniform Girls: The Fruit Is Ripe (1981)

Three high school student girls are on their spring break. They are expecting to get some sex of course. One becomes an adult magazine model. The second one nets a boyfriend. The third one is a little shy and needs helps from her friends. This last girl has fallen in love so her friends take her to his window and, by chance, arrive when mother is performing fellatio on son. The shy and pure third girl is in awe of a mother 'eating' genitalia and wants to give a similar performance.