You enroll an iPad when you open the Hub app and login using the enrollment credentials. For most iPads, they are enrolled when they are initially deployed and their enrollment doesn't ever change. However, if you ever perform a Device Wipe, you will need to re-enroll the iPad.
Each school is assigned a four or five character abbreviation for their school name which is used in the enrollment account. For example, enrollment accounts for student iPads at Good Shepherd Catholic Academy are: GSCA-STUD001, GSCA-STUD002, GSCA-STUD003, etc. And teacher iPads are enrolled with accounts: GSCA-TEAC001, GSCA-TEAC002, GSCA-TEAC003, etc.
Each enrollment account can only be used to enroll one iPad. If you try to enroll an iPad using an account that has already been used to enroll another iPad, you will receive an error which states: "The Operation couldn't be completed." If you perform a Device Wipe on an iPad, that enrollment account is "released" and can be used to enroll that same iPad or a different iPad.