According to the link at the end of this post, there are 1831 public schools in the state of Oklahoma servicing almost 630,000 students and likely several hundred thousand more employees. To protect that number of people and schools would require really REALLY big underground shelters. Think shelters larger than the lunch rooms or auditoriums these schools currently have, only buried underground and equipped with emergency ventilation and all that normal shelter stuff.
For simplicity, lets pretend it would only cost ten million per school. It would cost more, but let's pretend and keep it simple.
If it only cost ten mil per bunker the total cost to the state would be $183,120,000,000
That's 183 BILLION. If the state spent every penny they collected, just blew off every other obligation, it would still take them two decades just to pay it off. And all to mitigate a tragedy so astonishingly rare that it's international news when it happens. Note I didn't say prevent, because these shelters wouldn't do that. It takes time to pack a thousand people into a bunker, and the warning for tornados isn't necessarily that great.
Want to know why schools didn't build nuke shelters in the fifties? That's your answer. Money.
http://oklahoma.educationbug.org/public-schools/