If you monitor the nation’s media outlets for school bus news, as we do here at SBF, you’ll occasionally see stories about students being left unattended on school buses.

These incidents are relatively rare, considering that more than 25 million children in the U.S. are transported daily on yellow buses. But when they do happen, they put kids at risk and often lead to termination for the school bus driver.

Over the years, we’ve reported on many incidents of students being left stranded on school buses. We’ve published feature articles with tips on how to prevent these incidents — in fact, there is just such an article in our new October issue (“10 Tips to Bolster Bus Checks for Sleepers”).

In some cases, children are left alone on a bus for multiple hours. These incidents are traumatic for the child and his or her parents, but they typically haven’t resulted in serious physical harm to the child.

Tragically, the outcome was much worse on a school bus in Whittier, California, on Sept. 11 — already an infamous date. As previously reported, Hun Joon “Paul” Lee died after being left stranded on his school bus on a day when temperatures climbed into triple digits in some areas.

As of this writing, the autopsy on Lee has not yet been released, and officials have not yet confirmed exactly how long the autistic 19-year-old was left on the bus (his family members have told local media that they believe he was stranded for about nine hours).

What is clear is that the school bus driver did not find Lee on the bus when he should have — in a post-trip check of the bus.

There are various measures that school bus operations can implement to ensure that students aren’t left behind on their buses, but none of these measures is foolproof. The key is to implement multiple measures — to build in redundancy.

For example, drivers can be required to conduct two checks per run: one when the bus becomes empty and another upon returning to the depot. Electronic child-reminder systems can be installed on buses, or simpler devices like a “BUS EMPTY” sign can be hung in the rear window of the bus. Managers can issue frequent reminders on the topic, and they can be visible in the yard when checks are being done.

A spokesperson for the Pupil Transportation Cooperative, the operator of Lee’s bus, told reporters in a news conference that the agency is enhancing its safety procedures, including having two staff members check each bus for students and implementing an electronic child-check system.

As Wave Wire Services reported, the spokesperson called the tragedy “a teachable moment for this organization.”

In fact, the tragedy can be a teachable moment for all school bus operations. Lee’s death can serve as a poignant reminder of the importance of thoroughly checking every school bus after every run.

As a nation, we’ve already committed to “never forget” the terrorist attacks of Sept. 11, 2001. Now, the pupil transportation community must never forget the death of Paul Lee on Sept. 11, 2015.