DNS Auto-Default Software??

Is there something that can change back into your network's default DNS if someone changes the DNS?? If so, how do I bypass the software? I used to use Hotspot shield on my schools internet to bypass vpns and change my networks DNS to make Hotspot Shield work. Now,whenever I change the DNS, the wifi stops working. When I go to network diagnostics, it says that there is a problem with the DNS settings in Network preferences and tells me to use the DNS settings provided by your DHCP server. I change back into the default DNS and I works but i cant use vpns then. Need Help!!