If you have a building access control system just add the students to that, they clock in and out of rooms (most access control systems don't just control locks

If you have a building access control system just add the students to that, they clock in and out of rooms (most access control systems don't just control locks, they are widely used to clock staff in/out of work). There are solutions that use updated and rotating keys to combat cloning when phones are used instead of RFID/NFC cards. QR codes can be sent as photos or even live streams if you have short validity so you can't guarantee proximity - but RFID/NFC will except someone is clocking other students in. That would be easily noticeable if the readers are within the view of teachers, staff and other students. On many phones RFID/NFC also works just fine without having to open up an app first.