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As was mentioned, location can be a big factor for range. Part 15 AM range can be enough to cover the HS grounds around here. I used an AM transmitter for demonstration at a college and found that with the antenna inside the building the signal could not be heard down the hall so my first suggestion is to place the transmitter and antenna outdoors.
The transmitter should be connected to an earth ground and even a simple stake driven into the earth will help. The AMT-3000 has internal loading coils and needs to be tuned according to the instructions.
As your club gains experience with this they may want to try a more advanced loading coil/antenna system such as one with ground radials and an external loading coil. It is best to experiment to find what works in a given installation.
Here is a post where I describe my AM antenna installation. The buried radials are not seen but you can see the coil and transmitter enclosure.
Let us know of your progress.