Erase iPad

So, we operate a small School of Worship where the students use iPads for there course work. All of the iPads are owned by the school unless the student decides to purchase it. We recently had a student leave on bad terms and he did not give us the password or iCloud account information so that we could erase his iPad. We tried to reset it to the factory settings by holding down the home and lock button while connected to iTunes but it is still requiring his iCloud account info to complete restore it. Is there anything we can do to override or bypass this? Also, is there a way to prevent this issue in the future?