This article reports, that in a growing number of districts, school leaders are ranking the threat of sex offenders-some of them the parents of enrolled students--as one of their chief student-safety concerns. That threat, they say, is a primary reason for investing thousands of dollars in a Web-based tracking system that tells school officials if a parent, volunteer, vendor, or any other visitor is a registered sex offender. Setting up the system costs roughly $1,200 per school, and then about $400 a year to stay connected to the sex-offender database. One education lawyer said the growing popularity of such technology is a sign that schools are taking on a law-enforcement responsibility, or must do so. Districts want to treat this as a pre-emptive matter, and anything that helps them do that will be of great interest. The last thing they want or need is for something to happen and be surprised. Such tracking systems are likely to catch on with more districts as word spreads.