Movie with a number about the foreign legion?

This has been bothering me for some time.

I think this would have been a movie made before 1970.  I think the cast were mostly younger people, maybe they were all supposed to be "teenagers" (but possibly played by actors in their 20s and 30s).  I think the main setting might have been a beach, or a beach was involved.  I am almost certain it was an English language movie with musical numbers.

Anyway, at some point the cast go to a cinema to see a movie.  The movie they see is about the French Foreign Legion (or at least about French soldiers stationed in a desert).  But then for some reason (maybe they don't like the movie) some of the kids start singing pop/rock music.  So it becomes a whole musical number, with the characters from the Foreign Legion movie joining in.  The line between the screen and the auditorium becomes unclear, so the characters from the movie and the "real world" can interact with each other, and I think people from the movie come out into the cinema and vice versa.  At one point in the movie one of the kids (the lead singer, maybe) crosses his guitar with the rifle of one of the soldiers in the movie.

I think the movie was in black and white, but maybe I was just watching it on a black and white TV.