Authorities say all three are in good condition.
Emergency Medical Services Authority spokeswoman Lara O’Leary says about 10 people were injured at Centennial Elementary Tuesday and while three were taken to hospitals - others were treated at the scene.
Those transported include at least one 3rd-grader. No ages were released.
Mustang schools spokeswoman Shannon Rigsby says the injured were on a wagon that overturned when horses pulling it took off running after a piece of the wagon broke and struck the horses.
Tuesday was the anniversary of the land run that opened what is now Oklahoma to settlers. Numerous schools hold events on the anniversary.