Maybe....
so a faraday bag, an rfid cloner, a hapless victim, and some time and you could show easily how to circumvent the system.
think https://www.sparkfun.com/products/10070 would make a faraday bag?
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No, you need them to go lower than 400 ft or you need a concealed yard...
we need a dog pod grid.
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so what you're saying is, I need a 400ft tall fence...
To the orginal topic of RFID tracking IDs...there's already who product lines to protect you when outside of the area you're supposed to be scanned in
http://www.skymall.com/shopping/search.htm?query=rfid%20blocking%20wallet
Still, in the case of children...hate to think of people making their own scanners to track these schoolchildren outside of the school. RFIDs are generally passive response, they don't care what pings them. Unless they are scanner classroom doors and main entry doors, I assume these are the kind of RFIDs with a few dozen yards of range vs a few inches used on most security doors.