Tell Tales Out Of School Meaning

(idiomatic) To reveal confidential or sensitive information; to gossip.

Example: 1857, William Makepeace Thackeray, The Virginians, ch. 8:
  Mr. Washington had seen the gentlemen of honour and fashion over their cups, and perhaps thought that all their sayings and doings were not precisely such as would tend to instruct or edify a young man on his entrance into life; but he wisely chose to tell no tales out of school.
1871, Louisa May Alcott, Little Men, ch. 11:
  [T]he boys laughed and nudged one another, for it was evident that some one told tales out of school, else how could he know of the existence of these inconvenient treasures.
1884, Martha Finley, Elsie at Nantucket, ch. 9:
  "But now maybe I'm telling tales out of school," he added, with a laugh. "I shouldn't like to get the little girl into trouble."
1908, Lucy Maud Montgomery, "A Millionaire's Proposal" in Short Stories: 1907-1908:
  "Don't blush, Katherine, I am sure Mr. Willoughby won't tell any tales out of school to your old Valleyfield friends."
1997, Stephen King, Desperation, ISBN 9780451188465, (Google preview):
  "He wound up in the emergency room three different times, twice in Connecticut and once down here. The first two were drug ODs. I'm not telling tales out of school, because all that's been reported—exhaustively—in the press."
2004 April 5, Peter Bailey, "Reading Material," Time:
  Bush's former Treasury Secretary, Paul O'Neill, told tales out of school, prompting hot denials—and an investigation of whether he disclosed classified information.