PA28 Flight Instructor reported fuel exhaustion resulted in a forced landing in a field without damage or injuries.

Narrative

Took off from ZZZ with full fuel (48 gallons usable) and made an uneventful flight to ZZZ1. Before departing ZZZ1; checked the fuel level and estimated 30 usable gallons on board based on the previous flight's fuel burn and visual inspection of the fuel tanks. Departed ZZZ1 uneventfully to return to ZZZ and experienced an engine failure 5 miles northeast of ZZZ immediately upon switching fuel tanks. Instructor took control and attempted to troubleshoot; including switching back to the previous working tank to no avail; and subsequently executed a forced landing in a dirt field. No injuries or aircraft damage occurred. Visual inspection of the fuel tanks revealed no fuel remaining in the tanks. Total hobbs elapsed was X.X since receiving a top off (50 gallons) at the original departure airport. Instructor suspects a fuel leak since reaching fuel exhaustion after X hours would have required a fuel burn of approximately 16 gal/hr when the normal planned burn for the aircraft type is only 10 gal/hr. Maintenance findings thus far have been inconclusive.