William Castle in living color! 13 Frightened Girls is really NOT a horror film so be forewarned. It can be best described as Patty Duke meets Scooby Doo: a mixture of comedy and mystery/suspense that doesn't do a great job of either. The plot is predictable and offers no surprises. It's supposed to be campy but it's sense of humor doesn't translate very well to modern viewers.
On a positive note, 13 Frightened Girls is well acted and well filmed. Relatively unknown actress Kathy Dunn does a nice job as Candace "Candy" Hull; a teenage super sleuth with a heart of gold. There are also solid performances from Hugh Marlowe [The Day the Earth Stood Still, All About Eve] who plays Candy's father, as well as veteran TV actor Murray Hamilton [The mayor in Jaws, Father Ryan in The Amityville Horror] who plays a real secret agent whom Candy is way too fond of [The "ick" factor is high here.]
If you like your mysteries super sweet, then 13 Frightened Girls is for you. If not, skip this one and opt for one of Castle's masterpieces: House on Haunted Hill or The Tingler. Where is Vincent Price when you need him?
For more info check out the film's entry in IMDB.