How do I prevent a system update from occurring? We have 150 new XOOM Wifi tablets (MZ604) that we are distributing to high school students in 2 weeks. The tablets have stock ICS 4.0.4 on them, unrooted. 

The requirement is the tablets run Adobe Flash Player 11 in order to interact with math and science curriculum that coincides with our textbooks.  Adobe Flash Player does not work with Jelly Bean, therefore we need to prevent the devices from going past the 4.0.4 update. The students can take the devices off-campus and use them with their home wifi connections, so doing anything to block the devices at the school network is not going to work.
There's an app for this. You can use it to install Flash Player on a Jelly Bean device.

You can also try installing Flash Player and then upgrading the device to Jelly Bean.