Arms on a school campus can halt school shootings and mitigate the damage
During lockdown situations, classes are often trapped and are essentially sitting ducks. Having a firearm allows the class to have a chance to get out of the situation if a shooter decides to target the class. School shooters are also more likely to back down if there are firearms present.
The presence of concealed carry laws in respective states has been effective in reducing crime rates and is indicative that aggregate crime reduction in all states throughout the US could be achieved through the adoption of concealed carry laws. There is also already an established efficacy in School Resource Officer presence, an armed first responder with the purpose of decreasing response time to threats. School staff who choose to carry, and are fully trained to do so, are well-equipped to reduce this response time even further, decreasing the probability of increased casualties in the event of a shootout while also deterring criminal activity on school campuses in the first place. 
Teachers do not receive the same training as School Resource Officers. They also do not sign up for the same liability/job description. Therefore, teachers should not be viewed as equivalent to School Resource Officers.