Authentication Problem

I am using Mac OSX Leopard, not Snow Leopard. I recently bought a second Macbook Pro 13" for school, because my 15" was too big and heavy. This computer is like a cuter version of my 15". Anyway, the school made me wipe Snow Leopard from the hard drive and installed their own image, loaded with iWork 2009 and some other good software that would have otherwise cost me lots of money. However, the problem is that, when I need to log in or need to unlock the screen and the computer is connected to a network other than the school network, it takes about 10 minutes to process my password for the login screen. If the computer is resuming from sleep mode and requires a password, it will lag, then it won't even log in. The screen just goes to sleep, and when you wake the computer it just shows the login screen again without an error message. Can anyone tell me how to correct this without modifying the OS? If something happens to the OS or some critical system settings, then this computer cannot connect to my school network. This is disaster, so please only stuff that works.