13 Frightened Girls! (1963) 
I feel like Mr Castle has duped me, while there were 13 girls only one of them was frightened!
Poster art and title led me to believe this was a horror film, but instead it was a silly crime caper and our sleuth was a 16yr old girl named Candy! Had this been made in the late 70s it would have been a Disney film starring Jodie Foster!