|Directional Antennas are the way to go. They will give you twice the distance and you can only get on them while on campus an not off. I would probably consider either setting up a special subnet or VLAN just for the wireless AP. This way you have control over how it connects to your mall. Definitely firewall it and setup IDS and you might consider setting up certificate authentication so that only students can access.|
The range will depend on the manufacture you go with and the antennas you use so sorry can't help you with this. I would use something like NETSTUMBLER or ssIDer to check the signal around the campus.