Quoted:
You can technically carry at a school as long as there is no sign stating it.
Sorry but that's wrong. (unless you're referring to colleges or universities)
CRS 18-12-214, section 3A
(3) A permit issued pursuant to this part 2 does not authorize a person to carry a concealed handgun onto the real property, or into any improvements erected thereon, of a public elementary, middle, junior high, or high school
ETA: Beat to the punch