There are some options, but if the hardware is existing it depends on what has been installed. Some of these would be harder to implement with existing hardware than others:
Dogging (non-fire-rated doors only):
To unlock the door, the latch on the panic hardware is held back - typically this has been done with a hex wrench or key. Some schools have now gone to a thumbturn to undog (and lock) the panic in an emergency. There is also a product available that is basically a push button on the end of the panic bar. Dogging can also include an indicator that shows whether the door is locked or unlocked. The door always allows free egress - the door is only being locked from the outside.
Locking Outside Trim:
If a door has an outside lever handle, some panic hardware has a function that can lock the outside lever from the inside - similar to a classroom security lockset. This may also be available with an indicator to show whether it is locked. In a bank of doors, all of the doors except one could be kept locked on the outside, so in a lockdown situation only one door has to be locked. Or all of the doors can be kept locked on the outside but that means someone has to open the door from the inside every time someone wants to enter. Again, the locking is occurring in the outside lever - the panic hardware always allows free egress.
Electrified Hardware:
Where budget allows, there are a few different ways to lock the door with electrified hardware. I prefer not to use mag-locks, but the dogging function or the outside lever can be electrified.
ADAGuy asked about accessibility...in my experience, the key or wrench used for dogging or locking/unlocking is not required to meet the ADA standards, but if a thumbturn is used it should be operable without tight grasping, pinching, or twisting of the wrist because it's an operable part.