From PC Magazine:
How it works is fairly simple. Students with four or more unexcused absences in a year—approximately 75 are enrolled in the Anaheim test–are given handheld GPS devices instead of detentions or prosecutions. To make sure that said students are in school when they should be, the students are required to check in using the devices during five preset intervals: When they leave for school in the morning, when they arrive at school, lunchtime, when they leave school, and at 8 p.m. each day.
There’s a little more to it, so hit the link for the info.