HA! Found it!

Heisser Sommer (Hot Summer)

1968, color, 97 min. Feature, Musical
Dir.: Joachim Hasler
Script: Maurycy Janowski, Joachim Hasler
Camera: Joachim Hasler, Roland Dressel
Music: Gerd Natschinski, Thomas Natschinski
Cast: Frank Schöbel, Chris Doerk, Hanns-Michael Schmidt, Georg-Peter Welzel
VHS-NTSC, English subtitles - available for sale: defaorder@german.umass.edu
DVD, English subtitles - available for sale: defaorder@german.umass.edu

Synopsis:

While hitchhiking to the Baltic, a group of twelve girls from Leipzig meets a group of eleven boys from Karl-Marx-Stadt. The two groups bet on who will get to the Baltic first. Naturally, the girls have an advantage over the boys when it comes to hitchhiking, so the boys have to make up for this blow to their egos. When the two groups meet up on a Baltic beach, various encounters and provocations come up, the latter, however, remain harmless. But there is not much group cohesion: couples form and withdraw from the group. Everyone is intrigued by the love triangle between Brit, Wolf and Kai. Actually, Brit prefers Kai but she is flattered by Wolf's interest in her - after all, he is a whiz at mathematics and has the reputation of being a Casanova. Provoked by her girlfriends, Brit spends half the night with Wolf in the barn... The next morning, all the girls are against Brit and Wolf is looked at askance. But nobody knows what exactly went on in the barn. Brit, however, says nothing and Wolf's behavior is so overbearing that Kai fears that something dreadful might happen. A physical fight ensues between Kai and Wolf and Brit almost loses both admirers. Yet, there is a happy ending for Brit and Kai. An original DEFA musical, since its 1993 comeback in Germany, Hot Summer has risen to the ranks of quirky and "alternative" cult films. East Germany's own unique version of Grease.