Every time a fourth-grader passes through Rikkyo Elementary School's front gate, a small, gray plastic tag tucked inside his backpack beams a message to a computer in a nearby office.
The students are oblivious, but the computer logs the time they enter and leave, and a security guard watching the screen takes note. Moments later, their parents receive confirmation by e-mail.
In Japan, high-tech tagging has made the jump from grocery stores to the schoolyard.
Rikkyo officials hope the Radio Frequency Identification technology will serve as an early warning system for children who go missing.
"This won't prevent crimes against children," said Tsukasa Tanaka, principal at Rikkyo, a private school for boys in Tokyo. "But without the tags, we might not know that a student hadn't made it to school until we take roll. That's too late."
A handful of high-profile child killings have shocked low-crime Japan, prompting Rikkyo to look into several types of electronic monitoring.
The school, one of two in the country testing RFID tags, chose them because other technology such as satellite-based tracking would have betrayed too much information about students' whereabouts.
With the tags -- about the size of small key chains -- officials and parents will know if a student is late for school in the morning. Parents will also know if a child takes longer than usual to get home.
Like many schoolchildren in Tokyo, Rikkyo's students can spend as many as two hours getting to school by themselves on busy trains and subways. The school bans mobile phones, and parents wanted more assurances after slayings at a Japanese school in 2001 and recent kidnapping threats against one of Rikkyo's students, Tanaka said.
"I think the tags are a good idea because my two sons almost never leave school together," said Kimiko Shino, who has one son in second grade and one in third.
Shino said she has no worries that the tags could violate her family's privacy.
"Now I'll know what time to expect them home," she said of her sons, whose commute to Rikkyo takes 30 minutes.
The tags, developed by Japanese semiconductor and computer maker Fujitsu Ltd., use a technology that is beginning to gain widespread acceptance globally.
Retailers and delivery companies use RFID to keep tabs on merchandise. Motorists with prepaid RFID cards zip through traffic toll gates without stopping. Delta Air Lines plans to adopt an RFID baggage-handling system at every U.S. airport it serves.
The U.S. Food and Drug Administration has urged pharmaceutical companies to tag drugs to cut down on counterfeiting. Wal-Mart Stores Inc. has said it expects goods from 100 suppliers to incorporate the tags by January.
And in Japan, sushi restaurants rely on RFID to ensure that raw fish left out too long on revolving conveyor belts is replenished.
While critics say proposals to embed RFID chips in driver's licenses -- the state of Virginia is looking at the idea -- would violate individual privacy, the technology continues to get even more personal. Mexico's attorney general said this year that he and his staff were getting microchip implants for access to secure areas of their offices.
Although Japan has a low national crime rate, its schools are more security conscious than ever, still reeling from the shock of the 2001 school attack, in which a 38-year-old man stabbed eight children to death and wounded 15. The high-profile kidnapping and slaying of two young boys in northern Japan this year served as a reminder to parents and teachers that they can't be too careful.
Most Japanese schools now lock their gates and dispatch teachers on campus-wide security checks. Many show films and display posters reminding children to be wary of strangers, or teach students basic self-defense.
Some post private guards at gates or use closed-circuit cameras to keep tabs on students and visitors. A few have gone a step further, buying mobile phones with embedded Global Positioning System technology so parents can track their children all the time.
At first, only 40 Rikkyo fourth-graders had the tags, but all 718 students were scheduled to receive them.
The RFID tags contain tiny computer chips linked to small antennas. Radio-wave transmitters near the school's front gate read the chips, which have unique signals for each student. Antennas send information -- the exact times and the frequency with which the children enter and exit -- to a computer that e-mails the data to parents' mobile phones.
Fujitsu's Koichi Yamakawa said the system is designed to read as many as 100 students' tags simultaneously.
The system will eventually serve another purpose: Anyone entering the gates without a tag will trigger an alarm.
Rikkyo officials say the tags aren't cheap. Fujitsu is charging the school 15 million yen ($136,000), which includes the 3,500-yen ($31.80) per-tag cost. But Tanaka, Rikkyo's headmaster, said it's worth the expense.
"The school has to do all it can to ensure the safety of the children," he said.