Driveway motion sensor work ok except have ambient light adj for disabling during daylight so adjustment needed.
To replicate the design with low voltage, you need several 100mA pulsed IR senders and a diffused wide angle IR detector.
The pulses are sent and detected by separating the light inside so only outside reflections are strongest. Then the level is high gain AC amplified and then peak detected. This DC is then rectified again for variation or a Photo Receiver chip with AGC on the same wavelength (e.g 850 nm) is bridge rectified for change in signal due to motion. A window comparator circuit with two open collector outputs are joined so that either a drop in refelction or a rise means motion has occurred.
The response time depends on the low pass filter so use 10 seconds or 1 minute with plastic film caps and 10Meg Rs to achieve .if the teacher is motionless, the lights will go off, so normally a CD4060 5 minute timer , reset by motion ( inverted window detector low = reset high ) is all you need.
Make sense?
The IR senders are cheap 1V 100mA 5mm IR diodes in series with R to 5V to limit current.
The rep rate can match a TV Rx chip at 22kHz or whatever with AGC works well up to 30m with analog AGC out . Ac amplify this to window detector. Use a fresnel film lens or similar plastic to disperse light with low ref
Internal refection if IR Rx is not black, (daylight blocking filter) it will pick up FL lamp flicker.