(1978): A Danish short film produced by Filmlaboratoriet A-S. Sexy Schoolgirls (1978): Listed as an adult short on IMDb .
While released a year later than requested, this title is the closest literal match. It was directed by Gérard Grégory. Sexy Girl (1978) mp4
Below are the most likely matches based on 1978 cinema records: (1978): A Danish short film produced by Filmlaboratoriet A-S