Sheltering:

The obvious would be cots / blankets / privacy screens (these can be rope and curtain) and consumables especially water. The often overlooked would be eyeshades / ear-plugs as sleep aids, some cute stuffed animals to quiet the children some form of administrative protocol to adjudicate conflict among those being sheltered. Students and PTA can be called upon to supplie some of these items. e.g. cute stuffed animals (these must be new), old (but clean) bed sheets can be used for privacy screens, eye-shades and ear-plugs can be donated. PTA volunteers can be inducted for shelter management.

Comms:

Coordinate with your local HAM club, they are often looking to get involved with schools as an outreach activity of their own. ARES volunteers could setup a good 2 meter or even HF rig quickly if the antenna was in place ahead of time. This would provide you with the ability to communicate out of area if the police were too busy to handle health and welfare messages for you. Also local responders may be overwhelmed and unable to respond or monitor your FRS / GMRS radios. Being able to reach out of the disaster area is very helpful.

Community involvement:

Get the PTA and the students involved. After all they will be the ones hunkered down in your safe-tunnels. They are also a great source for donated material. Have students / Parents supply and stock Emergency desk supply kits - (a liter of water, power bar, light stick, whistle, warm windbreaker, comfort item) California has a specification for the student quake kit somewhere. Have parents help in supplying your shelter facility. Have fundraisers for needed expensive stuff such as cots - water storage - generators. Have students and parents participate in practice emergency scenarios. Use the less threatening ones such as tornado rather than terrorist - the drill is very similar and it will get them used to it. You use fire drills don't you?

Try to get the scenario to run with and without local government. You may be completely isolated with only those on campus.

Facilities:

Get some porta-poties. If your plumbing is out and you have half the town in your tunnels / gymnasium for a week you will want them. The stadium is a useful place for setting up recovery administration after the storm if you have some large tents. Remember, since you are listed as a shelter facility you may have many extra people looking to you beyond the normal population of the campus. Determine a potential location for a temporary cemetary and have shovels or equip your stadium tractor with a back hoe attachment. If there are large numbers of casualties you won't want them festering in the sun and you may not have electricity for the walkin fridge or access to the morgue.

Earthquakes:

Since you are vulnerable to these you can't count on sheltering inside. Have some tents on hand.

Colombine:

Check with local regs but you might want an armed staffer. If your local sherrif department takes volunteer deputies that may be a way to accomplish this. Have someone on staff become a deputy with a carry permit. Don't advertise this to anyone those who need protection will freak out! Just do it. Also get more staff (all if possible) trained on handling major trauma. This might just be first-responder. You may be able to get the Red-cross to use your school for holding classes and as a kick-back get them to train the staff for free to the first-responder level. If you have a volunteer deputy and they bring a weapon on-site no one should know -ever!(unless you have to stop the fool kid with the uzi that they brought from home.)
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Work to resolve social and home issues rather than trying to teach the kids not to shoot people. If some kid is bad enough off to kill himself or someone else knowing that they shouldn't have a gun is useless. Attack the problem not the symptom
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EMT:

Work with the EMT to ensure that they have all the supplies that they would have in the ambulance to stabilize and treat casualties. Have, at least, 1 if not more AED on hand. Encourage them to upgrade to paramedic (paying the tuition would help). Encourage them to take Wilderness first aid also as this course considers stabilizing individuals who may take a while to get to definitive treatment and that is often the case in disaster situations. Encourage more staff to get this training - you may be dealing with large numbers of casualties and One EMT can help one patient.

Have everyone involved in administering the disaster scenarios understand the Mass Casualty Triage protocol. The worst thing that an EMT can face is hysterical, poorly trained, bystanders questioning their decision that an individual with an arterial bleed in late stage decompensated shock must be abandoned in order that they may aid someone who has a chance of making it.