No, it's not, literally no one is expecting teachers to carry guns unless they want to.And when a job interview involves the question "would you be willing to carry a firearm to protect the students"? Or when you end up ostracized as the only teacher not willing to carry at a school? Or when the unfortunate event actually happens and you are one of the few not carrying?
Allowing teachers to carry is a way to shame or create doubt around the ones not willing to do so, much in the same way as teachers are widely expected to pay out of pocket for a range of school supplies. It will end up becoming common practice, despite the claims that it is voluntary.