Q: How can I unlock an iPhone 5s after I forgot the password?

See Forgot passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

If you don't know your passcode, you can always use Find My iPhone in a web browser and set a temporary passcode by putting the phone into lost mode

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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