My old school Porthcawl Comprehensive, in South Wales has introduced a SMS-surveillance system to stop truancy, with the delightfully honest name of “Informer”, that enables the school to text parents if kids are not present at class:
“Porthcawl Comprehensive School has had an Informer system for over a year.
Head teacher Kenneth Dykes found that while not all parents were enthusiastic, the system has helped improve communication between school and home. He said, “Being able to communicate with parents to let them know that their child has not attended registration has helped us increase attendance and keep in touch with parents more often.”
I look forward to reading how the kids in Porthcawl Comp inevitably hack their way around this…
Petafloptimism 
If the school wants to raise some money, it could also use the database of mobile numbers to make some money off the parents. For example, by running one of those ‘Congratulations, you have won a prize, phone 09xx xxxxx to claim it’ scams.
Now there is a really easy way to get around that, so long as you go into regester. Make yourself a note excusing you from lessons but sign it with a signiture the office has never seen before, but a legal gardian, like my Dad for example has never signed any of my notes or anything, just my mum, so i use his! ^__^
a simple way is to get round this text thingy is have a letter saying that u may sign out early and photo copy a signature (from our homework diarys because our parents have 2 sign it every 4 some reason). simple.