Let's make this easier.
Follow these instructions, from your account:
1: Launch the Command Prompt by typing "CMD" without the quotes into the start menu search box and pressing Enter.
2: Type in "net user Administrator *" without the quotes and press enter..
3: Log off.
4: Press Ctrl+Alt+Del*2, this should bring up a login prompt like you see at your school.
5: Type in administrator in the above box and press Enter to log in (No password required).
6: Go to Control Panel.
7: Go to User Accounts.
8: Go to Change your password.
9: Type your own password in (Do NOT include the password itself in the hint).
10: Click Change password.
Tada, the administrator is yours and yours only. Oh, and you may want to prevent them from accessing your controls:
1: Launch the Command Prompt by typing "CMD" without the quotes into the start menu search box and pressing Enter.
2: Type in "net user" without the quotes and press enter.
3: Disable ALL users (Except Administrator and ASPNET) via the command "net user username /active:no" without the quotes and replacing the "username" with the user names you happen to have in there.
3a: To check wether the account is already disabled, type in: "net user username" without the quotes and then hit enter, replacing the "username" with the name of the user you want to check the status on.
3b: When the list shows up, look for Account active, it should say "Account active: No" if the account is disabled, "Account active: Yes" if it is enabled.