A DevAddr is not unique: How does a network know a received packet is for them?
Also, accounts don’t generate traffic, but devices do. Even transmissions of devices that are not registered with TTN at all, will be received by TTN gateways, and will show in the gateway’s Traffic page in TTN Console. Even more: even bare LoRa packets, which are not using LoRaWAN at all, will be received and be shown. (Clicking an item in the Traffic page will show some more details.)
So: why do you think the packets are originating from a device of one of your students? And why do you worry about seeing unknown traffic?