C152 Flight Instructor reported they flew over another aircraft which had taxied on to the runway.

Narrative

My student and I made all appropriate calls in the pattern for Runway XX. We did hear on the radios someone was asking for a radio check on the ground. Someone acknowledged they could hear the guy in the DA20. We were on final and saw 2 planes waiting for departure for XX. As we were coming over the runway; we hear screaming going on. The guy in the DA20 has now pulled out behind us on the runway as we were passing over very low. I took over the controls from my student as it startled her and landed not realizing my mic was hot and said [expletive]. The guy in the DA20 now realizes he's on the wrong comm and is apologizing. As he's sitting on the numbers; he's troubleshooting his comms and the plane that was behind him holding short is telling him to get off the runway and screaming you entered the runway. During this ordeal there were also 2 planes on downwind too close to each other and one was telling the other they were too close. So; there was a lot going on with improper spacing; making sure they were on correct frequencies. The DA20 never looked right/left obviously before entering the runway. He was most likely on atis frequency is what I'm guessing. Once he saw us; he realized he messed up.