The original book is titled Passion Flower Hotel by Rosalind Erskine, a pen name for Roger Longrigg. A reprint is available on Amazon, and copies are available on Ebay. There were two follow ups, Passion Flowers In Italy and Passion Flowers In Business. In the book, although the girls do offer their services on a price list to the boys' school they are actually quite shy about being seen naked by each other. When the first girl gets chosen to do a strip tease she is more concerned that the other girls don't watch than the boys watching, although she does feel empowered and turned-on by the boys watching. So the opening sequence and the semi-lesbianic shower scene are totally out of character to the girls in the book version of the story. Also, the scenes involving the girls spying on a couple having sex in the boathouse are not in the book. Presumably this was to get a sex scene not involving someone supposed to be under age. To read now the first two books are nostalgic, interesting and mildly, very mildly, erotic - I've not read the third. But for a schoolboy in the mid sixties (it was published in 1962 I think, but I didn't discover it until secondary school) it was very exciting! And like a previous correspondent, when I saw it on late night TV I was very keen to watch it. I was partly disappointed at the deviation from the book's storyline but nonetheless enjoyed the opening sequences! I recorded it, then bought a second hand VHS, then bought it on DVD years ago.
Thank you for the report, Boggle-Eyes. Very interesting and informative. I often read the original books of movies that I like, and I wouldn't have thought of these on my own. Best Wishes!