Password protect folder.

When I delete work from my mac, sometimes a message will come up to say you need your administrator password to make changes. Is there any way I can get this message to come up on one of my folders.

There is a folder which the teachers use on the server and it needs to be password protected so the students cant access it. I have tried password-protecting it with a disk image but when one teacher mounts it on their desktop no one else can open the folder.

I could do with a pop up to come up to ask for administrator password before the folder can be opened.

If it's a folder on the server then you can add a username and password login to the server.