Pilot reported the alternator switch was inadvertently moved to the off position; resulting in a depleted battery and ATC not able to receive the 7600 and 7700 transponder signals. The flight crew landed without communications.

Narrative

I was the CFI occupying the front seat of a Great Lakes Biplane for the purpose of providing aerobatic instruction to the pilot-flying; who occupied the rear seat. We departed ZZZ and conducted aerobatic flight approximately 7 miles west of the field; over the Pacific Ocean.The flight was normal until we began our descent back to ZZZ. Approximately 3-4 miles west of the field; our radio and intercom failed. I could no longer talk to the Pilot Flying and neither of us could talk to ATC. The intercom (and radio) were not 100% dead; but rather garbled to a degree that neither of us could understand what the other was saying--only every 3rd or 4th syllable was audible.I advised the Pilot Flying to maintain altitude and circle outside of the ZZZ Class Delta airspace while we troubleshooted. From my position in the front; I had no access to any radio controls; intercom controls; or electrical switches. Those controls are available only in the rear cockpit. By reducing engine power and yelling to the side; I was able to tell the Pilot Flying what to do. I had him turn the radio and intercom on and off. There was no change. I had him unplug his headset and tried to contact the tower with just my headset plugged in. That failed. I had him adjust the volume and try a different radio frequency (ATIS at ZZZ). That frequency was equally garbled. Because communicating via yelling was so difficult; these steps took some time to complete.By now; our fuel was low. I told the Pilot Flying to squawk 7600 on the transponder and fly towards ZZZ; maintaining an altitude above the 1;500 foot traffic pattern altitude. I then focused on the tower; expecting to see a flashing-green light gun signal that would indicate the tower had received our 7600 squawk and understood our intentions. No light gun signal was visible to either myself or the Pilot Flying.Because our fuel was low; I instructed the Pilot Flying to change the squawk to 7700. He did so. By this time; I had good situational awareness of other traffic in the pattern so I instructed the pilot to descend to TPA (Traffic Pattern Altitude) and execute a normal approach and landing on Runway XXR. The landing was uneventful. Once on the pavement; I instructed the Pilot Flying to continue his rollout to the end of XXR rather than exit on an earlier taxiway. I did so because exiting on an earlier taxiway would have meant crossing another runway (XXL) without contact with the Tower. By taxiing to the end of XXR; we were able to exit the runway without crossing another runway.Once the airplane stopped; the Pilot Flying noticed that the alternator switch had been moved to the 'off' position. He turned it back on and the radios/intercom were instantly functional. I advised ZZZ Ground of our status; asked them for a phone number; and called the controller to debrief the situation.Because the radios were not 100% dead during our in-flight troubleshooting; it did not occur to me that the ENTIRE electrical system was failing due to a discharged battery. The ammeter is only installed in the rear cockpit; as are the switches that control all electrical circuits. I focused on troubleshooting the radio itself because; with the garbled comms; THAT appeared to be the source of the problem. Unfortunately; the total loss of electrical power also meant that the tower never received our 7600 or 7700 squawks. They advised us during the debrief that our transponder had actually stopped transmitting much earlier in the flight; a few minutes before the pilot and I lost communication with each other and ATC. We did not have enough fuel to reach any uncontrolled field. All other controlled fields in the ZZZ1 area are busier than ZZZ and lie in much more congested areas. I had no reason to suspect that ATC wasn't receiving our emergency squawks; so I do stand by my decision to land at ZZZ without contact with the tower. It was the correct; safest way to terminate the flight.However; it SHOULD have occurredto me to tell the Pilot Flying to verify the alternator was on. I expected a loss of electrical power would result in a 100% non-functional radio/intercom--not an intermittent one that sounds garbled. This is my key takeaway: an electrical system failure; much like an engine failure; may not manifest as a black-and-white event. Without access to the switches and indicators; it is extremely difficult for the front-seat pilot to diagnose the problem in-flight.Finally; the alternator switch was likely bumped into the OFF position during a pilot-exchange on the ground. To speed up training; our crew chief decided that we would swap students in the rear seat without shutting down the engine (while the front-seat CFI held the brakes and flight controls). During the swap from my first student to the second student; the aerobatic harness in the rear seat likely hit the alternator switch; which is located on a horizontal panel by the rear pilot's left thigh.This error was left untrapped by me because I normally ask the rear-seat student to verify that the alternator is on just after startup. Because there WAS no startup this time; that habit pattern was broken and I did not ask if the alternator was on.