If your wireless connection suddenly stops working, it's highly possible that you or one of your students has accidentally turned the wireless card power switch off.  

Elementary D630 laptops - You'll find the switch by the headphone plug in jack

1 "off" position Disables wireless devices
2 "on" position Enables wireless devices
3 "momentary" position Scans for WLAN networks

Elementary Netbook Cart - The physical switch is on the left side, slide the switch back toward the broadcasting PC to turn it on.  The picture shows it in the OFF position. The Fn + F8 key will also enable/disable wireless if the physical switch is ON.

Library Cart Lenovo Edge 15 - The switch is a soft switch activated by the red Fn key + F8.