Missouri) how can a private school get emergency epinephrine for our first aid supplies? do we need to hire a licensed nurse, or can general staff be trained in its use & able to purchase it for the school?

You should contact your local health department for guidance. State laws govern the who/what/why in providing any prescription or other medication to patients of any age or location. Perhaps you have a physician parent involved with the school that could follow local rules on delegating authority for stocking this drug & emergency by specified personnel.