Usually schools do this to protect licensing/sales associated with the logo. In our case, my daughter goes to Glendora H.S. (CA) which shares the Georgia logo/colors. When my wife buys logo'ed gear, she typically buys Georgia stuff because there is more variety and the prices are typically better. (for those that may complain, don't worry, I write plenty of checks to the school).

I don't know copyright/trademark law, but in a simplistic way, schools could actually profit by selectively allowing high schools and some other institutions rights to their logos.