I understand armed guards. Armed teachers, though? Even if the logistics and liability issues could be worked out, I’m skeptical.
There was an armed guard at Parkland - heck there was an armed guard at Columbine! - but it's kinda hard for them to have an impact when one person can kill 17 people in less than 5 minutes. Even with the argument that they might sometimes stop mass shooters from shooting more people, there's still the underlying issue that shooters can almost always rack up a massive body count before they ever get there.
Armed teachers, especially actively carrying, fails risk-benefit assessment so hard I have to assume people that are gungho about it either haven't considered any risks or have a vested interest in selling more guns.
There was only one guard at a school with over 3,00 students, and multiple buildings.
There has to be a limited amount of entrances into these schools(maybe 3 or 4), and should have an armed guard at each enterance. This could come from active/retired police/military.
You're right it only takes 5 mins to go on a killing spree, and it takes swat teams 5-10 minutes to even enter the building. To me this means you need armed guards to be able to stop, and alert at the point of entry .
Also should have cameras in the parking lot and entrance points of the buildings to be able to alert a lockdown as soon as possible.