It's not just used for tracking attendance, it's used for a lot of other things, like paying for lunch at the cafeteria. Conceivably it could probably be used to control access into school buildings, so only staff, faculty, and students can get in. For those of us who work for a living and carry an ID badge at work, this is more or less what we use them for.
Bar codes are really no better, since they can be scanned at line-of-sight for a remarkable distance. They're less reliable, which is why RFIDs were invented in the first place.
Through the wholly unremarkable combination of having a job, living in a city with a functioning public transit system I sometimes use, being a Zipcar member, and having a credit card, I end up carrying about five RFID's at all times. RFID's are part of the world, not magical antichrist technology that the evil schools are conditioning children into accepting. Schools are a few years behind the technology curve, actually, as they almost always are.