Those are pc's connected to the same network, are you running a VPN connection to Work? Do you live on Campus, are you using the schools network?
The best way to protect yourself against any intruders:
1.Make sure you are using WPA2 encryption for your wifi, although that won't help you against intruders from INSIDE the school network, think of your airport router as a extension for the schools internal wired network
2. Enable the firewall, system preferences---->Security----->Firewall---->(if the option start firewall is greyed out, click on the lock in the lower left corner, and enter your Password) Click on start Firewall, then on Advanced. there you can set the level of protection, if you can enable "block all incomming connections" , but this will also stop bittorrent from working!
also enable stealth mode
3. make sure you HAVE a password set on all your user accounts on your mac!
4. system---settings--sharing, make sure nothing is selected , if you don't want to share any files or basically anything with your "friends" on the network
5. if you want to feel 100% secure, use hotspot shield (
http://hotspotshield.com/) its a free VPN to some server in the US (better would be your OWN VPN at home,...) I use it everytime i am on a Wifi hotspot at starbucks
I hope I was able to help you