How do I block a network account in 10.6.8?

TL;DR - I have a local account with the same credentials as a network account. The network account logs in, but doesn't work the way I want. How can I get the login window to not ask me to pick between the two and default to the local account? Not allowing network logins under "Accounts" doesn't work.

I have about 15 iMacs running 10.6.8 that I am having an issue with. I am in a middle school and we're going to begin testing students soon. Our plan is to have a unique user account that students login to that will automatically launch the test in order to minimize potential problems. All of these computers are part of our Windows domain to manage printing, and the account we're using is a network account. I've set up 120 other computers without an issue - they login with the network account, hold all the settings and perform exactly as expected. The exception being three machines that for whatever reason refuse to login to the network account, though everything is identical to working machines. As a workaround, I created a local user with the exact same username and password. The idea is that staff won't have two sets of credentials when they want to start the test.

These 15 computers do not hold any settings for the network account. I've modified the dock, changed settings, and every time I log out it's all lost. I log back in, expecting to see my changes, and it's as if it's creating a new default account each time. So as a workaround, I created the local user with the same username and password as in the cases above. Now when I attempt to login, the login window shows me two options - the network account, and the local account. I've disabled logging in with a network account and it doesn't make a difference. I'm assuming that this is happening because the network account did/does successfully login, even if it's not holding settings, while the machines that never did login don't have this problem, thus no screen asking me to pick between logins.

This is going to confuse my users and cause problems that I may not be available to address within an approriate timeframe. Once a network account has successfully logged in, is there any way prevent it from trying to login again, defaulting to the local account instead? I know OS X is supposed to check against local users first, but I've never seen it ask me to pick between local and network like this before. Thanks in advance for your help!