The hardware approach may look appealing but hardware can be expensive to buy, maintain and is subject to failure. More importantly, hardware-based solutions only identify the vehicle, NOT the driver and is passive by design, passive in the sense that if there is an issue with a child getting into a vehicle with the wrong individual, school administrators can only be notified after the fact (usually by an irate parent).
When you turn to your hardware based identification system for answers, it can only tell (with certainty) that the child left in a blue Chevy Volt for example. But who was driving? Sure, the Tesla may have had an identifier the hardware recognized, but has NO CLUE who was behind the wheel. That's a problem and a sticky one. Parents often have to drive borrowed or rental cars or may have requested that a surrogate pick up children. In these cases the hardware solution fails.
Let's move away from school-owned hardware and see if we can build a better mouse trap. Most parents and guardians show up every day with a smartphone. Some solution providers create an app that allows parents to communicate their wishes for their child at dismissal so they don't have to overwhelm your front office staff with phone call, emails, hand written notes et al... brilliant! What's more, an app running on parents' devices can enable us to detect their presence when they arrive on campus using technologies like geo-fencing. Since they own their phone and we have issued credentials specific to each user, with the app 'open' we can now identify them with a high degree of certainty. Great, so let's have a 'check in' button appear on their app when they arrive for carline so we can identify the individual...identity problem solved.