Cessna 172 Flight Instructor reported electrical failure affecting different systems during training flight. Flight returned to departure airport and landed safely.

Narrative

During cruise flight after the ZZZZZ reporting point; I instructed my student to perform two 90-degree clearing turns in preparation for a slow flight maneuver. During the maneuver setup; I observed the turn coordinator displaying a red flag; indicating the instrument was inoperative. Shortly thereafter; both fuel quantity indicators dropped to zero; despite having departed with approximately 25 gallons total. At the same time; we experienced a loss of COM 2.Recognizing a developing electrical issue; I immediately terminated the maneuver; instructed the student to recover from slow flight; and initiated a turn back toward the departure airport. I referenced the appropriate checklist and confirmed that no circuit breakers were tripped. To conserve electrical power; I reduced electrical load by turning off all non-essential lights and equipment.While en route back to the airport; we subsequently lost COM 1; resulting in complete radio communication failure. I used my personal cell phone to contact ZZZ Tower; advised them of our position; intentions; and total communication failure; and requested light gun signals. ATC acknowledged via phone and cleared us to proceed directly to the airport.ATC suggested diverting to ZZZ1 Airport; however; I advised that there were no engine-related issues and that we had sufficient altitude and performance to safely return to our home field. I set the transponder to 7600 and continued navigation using visual references.Due to the electrical failure; flaps were unavailable; as they are electrically actuated in this aircraft. I executed a no-flap landing; landed safely; and cleared the runway. After clearing; I again contacted the tower by phone to receive taxi instructions. ATC advised that no further action or report was required.