Current thinking is that an attention tone (something different than the fire alarm or other types of signals) followed by a calm voice message customized to the location. The person recording it should be the principal or whoever else does the normal messaging. It should also include a procedural reference (i.e. "follow lockin procedure"; "follow lockout procedure" etc.
All of this is a bit useless without good policy, training, exercise and enforcement. I also agree with Corey about the messaging; but i would suggest that push notifications be employed along with the text.
Your customer should be very careful about who messages are sent to outside the school. Although parents should be notified, the content of the message should be different... least you have a mob of terrified parents show up at the school.
There are a number of websites that have script recommendations, including scripts for specific types of messaging. I would advise you to be very careful about the message content. If you are an integrator, i would let others make the decision and you just set it up for them. You may want to suggest that a "Committee" be appointed (they should already have one) to best ensure all minds are used in the decision. If the school has police or security, involve them as well.
You can facilitate, but should not be the decision maker. I think your attorney would agree with me.