Flight instructor reported while in the pattern they observed an aircraft on the Runway waiting to depart at an intersection down field and another aircraft on the Runway waiting to depart full length who was not aware of the other aircraft. Flight instructor advised the aircraft waiting full length of the other aircraft and they departed after the other aircraft departed.

Narrative

I was instructing in the pattern at ZZZ. My student and I were downwind abeam the midpoint of Runway XX. A Cessna 150 had just landed and taxied clear of Runway XX the active Runway. 2 Aircraft; a Vision jet and a C-172 were holding short to depart at XX [at Taxiway] 1 full length. The Cessna 150 taxied clear of the Runway and then proceeded to make a radio call stating they were departing Runway XX. At the same time the vision jet was taking Runway XX for a full length departure. I called to the vision jet on Unicom frequency to inform him someone was on the Runway at [Taxiway] 2 for departure. The vision jet responded and held position while the Cessna 150 departed. A discussion was had on the Unicom about the importance of stating clearly any intention of not making a full length departure. This aircraft spent their entire training session cutting off multiple aircrafts by doing short length departures cutting off the traffic in line for full length departures. Had I not been in a downwind position to call out this potential Runway incursion it could have been extremely dangerous for both aircraft involved.