I should have mentioned this. Smart Lock Enterprise has an Approved Browser built into it for webpage specific content. It displays approved websites (could be local content) but not other websites. In your case, even if someone put the tablet on the internet (via a hotspot or something) it would block all websites except the ones you have explicitly approved.

I have written some software called Smart Lock Enterprise that locks down Android devices to just one or a few (white list) apps. It works great on tablets and is in several production environments. You can find the consumer version at https://market.android.com/details?id=com.nightshadelabs.smartlock.pro.free and you can contact me about a business version at david [at] nightshadelabs.com