From the French release -
Mainly French names are used and these are probably not the original ones.
Cécilie (Julia) wants to make the most of her parents being on holiday. She tells her friend Karine (Karen) that she has a crush on Charles, her piano teacher, and hopes that he will be the one to make her a real woman. So when she turns the piano lesson into a "dancing" exercise, the governess, Greta, throws Charles out. Then her cousins Paul and Line arrive. At dusk, Cécilie watches them go to bed together and realises she is more attracted towards Paul than she was towards Charles. The following day, while the three cousins are out shopping, Charles appears and offers Greta a bunch of flowers and asks her to forgive him. But Greta wants more and gets it. Back to the villa, Cécilie seduces Paul while Charles cures Line of a headache in his own way and Greta teaches Karine how soft and nice a woman's hands and tongue can be!
From a review originally published in La Saison Cinematographique 1981, translated by Prophilo.
Helga Wild plays a spoilt bourgeois teen who nevertheless looks in her late thirties. This film is from Dorle Buchner's 'Siggi days' but the couple have no scene together and Dorle's performance is lesbian-only. Another interesting point about"Schoolgirls" is its producers' deliberate attempt to present it as an American production, with English credits and fake aliases.
Karl is played by Siggi Buchner.