This is perhaps the best known phrase in America. Over the course of the last ten years, this phrase has become the most popular way to end morning announcements in schools across the USA. It is probably the most common in elementary schools, but it is also very popular in middle schools and high schools.
The origins of the phrase are aggressively obscure, with it being credited to a number of principals, guidance counselors, and other local school personnel. The largest high school in Detroit, Detroit Lakes High School, proudly claims it as the school motto, and states that it was coined by former principal Steve Morben. One random person on the internet reports that his father learned it as a common Croatian saying. An overwhelming number of people simply cite "an old mentor" or the like.
Regardless of origin, it is fairly certain that it was popularized by Project Wisdom (link), a for-profit source of inspirational morning messages, each of which use this phrase as a sign off. Project Wisdom has been producing morning messages since 1992, but do not have any information as to when they started using this sign-off, nor where it came from.