I don't use it very much, no. in summary, it means to talk about things you don't shouldn't talk about. It may be you lack the authority to say anything officially, or it may be about things that might make an organization not look at good to members outside of the organization, or it may be an announcement that will be made officially by someone else later.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
In addition to saying something you do not have the expertise to say, or that you are not entitled to say given your position or relationship to the person or group to whom you're talking, it can also mean to give away a secret, or to broadcast something that's none of your business. More generally, "talking out of school" or "speaking out of school" is saying something you have no right to