If you've forgotten the passcode to unlock your phone then follow the instructions on this page : If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

If you mean that you've forgotten your account's password to reactivate the phone : http://iforgot.apple.com

There are two different locking mechanisms:  passcode, prevents access to your data,  and activation lock, prevents use of device if stolen.  You do not want the students setting the activation lock.

If the passcode is set & forgotten , you can reload from backup or wipe ( first typed whip ) the device.

iPhone or iPad locked from incorrect passcode.
If you enter the wrong passcode six or more times, you'll see a message that says Ipad is disabled.

See how to restore from backup.  No back up on your computer, click on the triangle before "If you have never synced your device with iTunes, or you do not have access to a computer"
http://support.apple.com/kb/ht1212

The problem arises when someone sets the activation lock which is set by logging onto your apple id then setting the appropriate option.   DEP is the preferred preventative.  It established your organization as the owner.  The student cannot set an activation log.

How password and activation lock work:

http://support.apple.com/en-us/HT1212

Undoing the Activation lock

iCloud: Activation Lock
Describes what the activation lock does:
http://support.apple.com/kb/PH13695?viewlocale=en_US

For how to remove, see Texas Mac Man post in
https://discussions.apple.com/thread/6528124

If you're asked for the password to your previous Apple ID when signing out of iCloudhttps://support.apple.com/en-us/HT203828

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Apple ID: How to recover your Apple ID

How to find and change your Apple ID password

https://support.apple.com/en-us/HT201487

iforgot.apple.com

http://support.apple.com/kb/HT5625?viewlocale=en_US

If you decide not to provide a rescue email address, you can't reset your security questions and answers yourself later in the event you can't remember the answers to your questions. You'll need to contact iTunes Store support to have your questions and answers reset.

Rescue email address and how to reset Apple ID security questions
http://support.apple.com/kb/HT5312
by Sheila M  [apple employee ]

You do want to  be the first to set the Activation lock.  This way you can at least restore you iPad should it get locked out.

Activation lock

Find My iPad

"With iOS 7, Find My iPhone includes a new feature called Activation Lock, which is designed to prevent anyone else from using your iPhone, iPad, or iPod touch if you ever lose it. It starts working the moment you turn on Find My iPhone in iOS 7."

http://support.apple.com/kb/HT5818