This is not only one of the best, but also one of the most important Italian gialli in that it forms a nexus with a two other genres of film that influenced or were influenced by the gialli. This was a West German co-production, nominally set In England, and (very loosely) based on an Edgar Wallace novel which connects it to the West German "krimi" series, a more obscure series than the Italian gialli but a clear influence on the latter. Stylistically, this colorful and garish film is much more of a giallo than a krimi, but influence is there, and it features Joachim Fuchsberger and Karin Bal, who were both popular actors in the earlier krimis, in supporting roles .
Perhaps even more interesting is the connection between this movie and the American/Canadian slasher films. Along with "Bay of Blood", "Schoolgirl Killer", "Torso", the Spanish film "La Residencia" and the British film "Assault", this was one of the European films that received the widest release stateside and probably had the greatest influence on the early slasher genre. This movie kicked off what could be called the "schoolgirl gialli". Unlike the other gialli, which focused on decadent adult European jet-setters, but like the slasher films, the "schoolgirl giallo" had seemingly innocent adolescent protagonists being picked off by a deviant, but often moralistic killer. But while the slasher movie victims were merely guilty of promiscuity and other typically irresponsible teen behavior, their schoolgirl gialli peers were engaged in behavior that one hopes even in Europe at the time wasn't typical--ie. teen prostitution, sex orgies, back-alley abortions.
Even though it was made years earlier, this movie is actually a lot sleazier, but also much better made than the vast majority of slasher films. The "hero" is a gym teacher at a private girl's college who witnesses one of the murders. The problem is he was schtupping one of his students in a rowboat at the time! It greatly helps though that this character is played by the very likable Fabio Testi. It also helps that none of the girls is especially believable as a "sixth former"--the victims, who are dispatched in very unpleasant (and full-frontal) fashion are too voluptuous to be believable as teenagers, while the two female leads--Spaniard Cristina Galbo and American Camille Keaton, were actually both in their early twenties at the time and at the very height of their very considerable beauty. Most importantly, however, this movie is very well-made and stylistic with an excellent musical score. Director Massimo Dallamano ranks right up there with Mario Bava, Dario Argento, Lucio Fulci, and Sergio "Torso" Martino as one of the true masters of the Italian giallo thrillers.
For what it's worth I personally own over 120 Italian gialli, and though I would be loath to rank all of them, I would certainly put this one in the top five.
