Ok, so I've just had a thought - I believe all you need to do is enable loopback processing in MERGE mode, and then assign a LOG ON script under the USER configuration and apply it to the computers.
So, if your OUs are divided into classrooms, and you want all users who are in a specific classroom to have a specific default printer, you would need to enable loopback policy processing for EACH classroom OU (can be done through a single GPO applied to a root OU containing the classroom OUs if that's how you have it set up, provided they contain only the computer accounts, and that the user accounts are separate elsewhere).
Once this setting is enabled in Merge Mode, you will then need to set up individual GPOs that apply the LOG IN script under the USER CONFIG settings to the same OUs.
So, to try and simplify/summarise that load of babble above (sorry I'm in a bit of a hurry!!), you need to first ensure loopback policy in MERGE mode is enabled for ALL the PCs in question. (Computer Config > Administrative Templates > System > Group Policy > enable the Loopback Policy option.). Then ensure that each separate classroom OU also has a GPO containing the relevant logon script (the script that has the necessary printer name in it!) applied as well.
Anyway, any questions whatsoever, just ask, and maybe if we break it down you'll have a better chance of understanding it. :)
Pete