School safety is a concern for teachers and students alike. Younger students may need to rely more on their teachers to protect them, while older students may be able to protect themselves in some cases.
I think knowledge would be my number one recommendation. She @Beth should take the USCCA Countering the Mass Shooter Threat course in her area, or at least online. There is an entire section of the instructional on schools and places of education regading safety.
I would start with some door blocking device something along the lines of these: https://fightingchancesolutions.com/
Also research as much as you can, sometimes information and knowing the best response is your best defense.
An AR500 Back-pack is the main product, I would be interested in Beth’s take on. I still think the most important thing for an older kid to protect themselves at school is freedom to act. Like anything else teachers are a mixed bag, I have let my kids know that they can act to protect themselves. They know the sound of gun fire vs fire works etc… if they think the teacher (or anyone else) is mistaken or not taking the situation serious they are free to get to safety.