Unfortunately there is no way to block access to this. However we did initially see an issue with students forgetting their iOS password, but that has since declined. We use casper to remove forgotten passcodes in certain situations but did not want to make it a get out of jail free pass for students each time they forgot it. Making them responsible for remembering the code if they choose to set one or loose all of your locally stored data by wiping the device seems to work well for us.
Actually there is a way to block access to passcode usage on iPads. We have tested it here and it works. What has to be done is two fold. First, in the settings for the pre-stage enrollment you have to remove the option for setting up the passcode. When the student goes through the setup process they will not see the option for setting up a passcode. Second, you need to create configuration profile and uncheck the box for " Allow modifying passcode (supervised only)" under restrictions and set the scope to the iPads you don't want passcodes used on. When the student has finished setting up the iPad the configuration profile gets downloaded to the device and removes the option for creating a passcode in the settings. So the option for setting up a passcode during the setup process is blocked and the option for creating one thru settings is removed leaving no way for the students to create a passcode on the device.