I have to assume that in the event of an emergency the school will keep the students together to shelter in place until arrangements can be made for pick up by parents or transfer to another reception center. I can't imagine the school just releasing everyone to make it on their own. You or your parents should ask the school administration about shelter and evacuation plans. Does the school have a plan? What is it? Does the school have food, water, sanitation, etc. for students if everyone has to shelter in place?

You should talk with your family about a plan for what everyone should do in the event of an emergency that catches everyone away from home.

As for personal preparation I would suggest keeping a small bag in your locker. My locker (a lifetime ago!) was small and stuffed with my other junk, but you should find room for a small bag. In that bag I would put:

flashlight with extra batteries
roadmap of the city/county
N95 particle mask
small first aid kit
water--as much as you can fit
food--bag of trail mix, energy bars
cash & coins

On your keychain I would carry a small keychain light such as the Photon II.

Your smart to be thinking of this. I hope your school administrators are as smart.