I have a locking pistol box in my truck. Or under the seat in my van. But at a school it's a no no even in the car. You would need to park off the property to leave it in the car.
You can get a locking box pretty cheap. I bolted mine to the floor behind the driver seat. You can bolt them in the trunk as well. I have heard that some folks lock them to the seat rails so they don't have to bolt them in. Lot's of options. A locking glove box would work too. I prefer not to leave mine loose in the car unless there is no other choice, ie in my van. I need to get a box for it.