how to hack the unknown person's wifi password:

How to show the hidden wifi password?
                                                          
Even if you could hide it they, being admin, could always restore the settings, see the password and hide the setting again without you even knowing. What you want (hiding something from an admin) is never going to work.
The correct solution to your problem is to make those students users and elevate privileges to what you want them to do that requires then to even need admin privileges. Like installing software, updating etc. Here is an example using policyKit: How can I set the Software Center to install software for non-root users?

1. Click the system status area on the top bar and select Wi-Fi.
2. Select Wi-Fi Settings.
3. Click the Connect to Hidden Network… button.
4. In the window that appears, select a previously-connected hidden network using the Connection drop-down list, or New for a new one.
5. For a new connection, type the network name and choose the type of wireless security from the Wi-Fi security drop-down list.
6. Enter the password or other security details.
7. Click Connect.
   You may have to check the settings of the wireless access point or router to see what the network name is. If you don't have the network name (SSID), you can use the BSSID (Basic Service Set Identifier, the access point's MAC address), which looks something like 02:00:01:02:03:04 and can usually be found on the underside of the access point.
   You should also check the security settings for the wireless access point. Look for terms like WEP and WPA        
8. thats all now you can enjoy with others internet in your pc or mobile