Masking Your MAC Address

Now that some other people in my school have started using there laptops, once in awhile get an error message saying "MAC Address in use" I tryed changing my MAC address with the following

ip link set eth0 address 00:00:00:00:00:00
ifconfig eth0 hw ether 00:00:00:00:00:00

Both times once I change the MAC address if I was connected to the network I cannt connect to anything anymore and if I do it before I connect to the network I cannt connect at all. What am I doing wrong?

Thanks
nuke