What you will need:
A jailbroken ipod touch, iphone, or ipad
You will need access to one of your school's apple computers or idevices (such as an ipad or itouch.)
Most school wifi networks work with a pre-programmed list of accepted MAC addresses. A MAC address is a unique signature most electronic devices have. If yours is on the list, you can use the internet. This will show you how to change yours to be the same as one of the school's.
The first thing you need to do is obtain a mac address from an apple device connected to your schools wifi network. If it is an ipod touch or an ipad, go to the settings app and click on general. Then click on about. Scroll down to near the bottom where it says: wifi address. There should be a bunch of letters and numbers separated by colons. Write it down EXACTLY how it appears, including the capital letters and colons.
If the only thing you have access to is one of the apple laptops, this is how to find the mac address. I've never actually used this method, so be sure to comment and tell me if it works. Go to system preferences, go to network, and then to advanced. Your MAC address is your airport id. There should be a bunch of letters and numbers separated by colons. Write it down EXACTLY how it appears, including the capital letters and colons.
Feel free to comment and ask questions, and I will try to get back to you as soon as I can.