A new dress code adopted in Arlington, Texas, prohibits low-cut blouses, tops and sweaters with "plunging necklines".
A simple sight test will be used to define the level of cleavage shown.
"If you can see the space between a woman's breasts, it's not allowed,"
James Adams, principal of Arlington high said.
Unlike their counterparts in the UK, American pupils are not required to wear school uniforms.
Sherri Wade, of Arlington school board, said: "It's gotten bad enough that, unfortunately, our young males are looking at more than their English book, their speech book, their science book.
"And it's kind of nice to have something left to the imagination."