The ip is set by your dhcp server. If you check on dashboard you see a ip for eth0 and also a mac address. The simple solution is to tell yout dhcp server to always assign same ip to that mac address. This works for all.
If not and you are using our pi image go to Global Setting->Network/Time->Wired Connection
Switch from automatic to manual and set correct IP that is not inside dhcp server dynamic pool and is not used by any other device in school, assign netmask and gateway. Then eth0 gets assigned that ip directly without asking dhcp. I would prevent this if possible as it always risks no connection on wrong config and ip clashes just because it is not controlled by your instance assigning IPs.
