Ubuntu – Network manager forgets wireless password after sleeping or powering off

Setup the wireless network as an admin, and check the box to make the connection "Available to all users" at the bottom of the dialog when you edit the wireless network.

Ok, so let me be more verbose and explicit:

1. Log in as an administrator. This account is not accessible by students.
2. Click the network icon and select "Edit connections".
3. In the Wireless tab, select your network and click edit.
4. In the bottom of that window, click "Available to all users".

Now, the connection will be configured automatically at boot, other users are able to use the network, but if they try to edit that network, they'll be asked to input your password. Failing to do that, means not being able to edit that connection, hence not being able to see the password.

If indeed, like you say, students can see the password when the connection fails, then that is a bug to be fixed, and not worked around. It's quite obvious that users should not be able to read other peoples personal information.

Click on the network icon in the panel and choose Edit connections... Go to the wireless tab, select your network and click edit. On the Wireless security tab, make sure the password field isn't empty.