What a geofence is
A geofence is a circle drawn around a point on a map. When a student clocks in, their phone reports where it is and how accurate that reading is, and the server checks whether it falls inside the circle. Done well, the reading happens only at that moment, after the student taps, and not continuously in the background.
How accurate it is indoors
Phones combine GPS, Wi-Fi and cell signals. Outdoors a reading can be within a few meters. Indoors it varies by building: a modern phone often reads within 30 to 100 feet, and thick walls or basements can do worse. That is why a good system looks at the accuracy the phone reports, not only the position, and treats a vague reading differently from a sharp one.
Setting the fence
Start by putting the pin on the classroom itself rather than trusting a street address, which usually lands on the building's entrance or the middle of a campus. A satellite view helps. AttendVia defaults to a 100-foot radius, then we suggest a walk test: stand in each corner of the room with a phone, see what it reports, and widen the fence only if you must. Too wide lets in the parking lot; too narrow turns away students who are in their seats.
Privacy and fairness
Tell students exactly when location is read and who sees it. Reading location only at clock-in and clock-out, and keeping only the distance for refused attempts, respects them and is also easier to defend. Always have a way for the instructor to mark someone present, because phones die and buildings block signals. Technology should support the person in the room, not overrule them.