Speaking "out of school" is making commentary that the speaker has no right to make. They may be commenting on issues over their head, are none of their business, or that cross the line in terms of what is generally acceptable. Talking out of school is assuming you know more than you possibly could about an issue.
I can say that my sister is a jerk, but you're out of school, crossing the line, disrespecting me by talking about her.
by PCMLR July 08, 2010