Enforcing a passcode on a device is possible with the use of smart groups. You will have to manually enter the passcode on the device if you want the passcodes to be the same on every device. Once a passcode is present, it should retain that passcode.
The way this is done is by creating two smart groups: one for devices without a passcode and one for the devices that receive a passcode.
The first smart group should contain the filter "Has passcode = No". This will add every device in your system that does not currently have a passcode. You add a profile to this smart group that requires you to set a passcode.
The second smart group should be the opposite of the first one. The second smart group contains the filter that has "Has passcode = Yes". This group will receive the iPads that you have entered a passcode for. In this smart group, you add a profile that does not allow the user to modify the passcode.
Once the device lands in the second smart group it now has a passcode that cannot be removed or modified, since it automatically gets removed from the first smart group.
Note: When you send in an iOS update to the device from Jamf School Management System, passcodes get cleared and you would have to repeat this process.