Cessna 150 Flight Instructor reported a minor tail strike on landing on a training flight with a student after student failed to apply proper rudder inputs.

Narrative

We had a student at our school with 100 hours and he was pre solo. As a new instructor; I came in to help fill time for another instructor & ended up taking this student in; as they only booked me from then on. I've flown with this student before in another type Cessna-172; and our landings were being worked on in that plane. As I was coming in to land for this landing; we had runway secured and student noticed they were drifting left of centerline. We knew winds were variable because some weather was soon to be in the area; but winds were very light still. I told student 'right rudder... right rudder' and input more also; as well as trying to make corrective actions with ailerons; and we kept drifting left; so I initiated a go around. We were at full flaps- 40deg; and just about to pull idle because runway was secure; and as I took controls and put full power; we just sank even more to the ground and at that point; hit the ground (which I wasn't aware of at the time because the plane was now not responding to my input controls). Unable to climb or gain airspeed. I then pulled full idle after realizing we were not going to get off the ground; and we landed; stayed on runway; and exited right away. Was able to taxi back to ramp for inspection. There were many factors that day; and the plane we were in was not powerful at all; 100HP; so I think trying to save the landing by doing a go around; made it worse; instead of landing off centerline. Everything was going right; until it wasn't. We tried to save the landing; and because a C150 is not powerful; a go around isn't always the answer; like they say. Going forward: Maybe in certain horsepower planes; making a go around be at a certain altitude and flap setting; no lower or less; could have helped. I would have tried to land and been way off centerline; rather than trying for the go around and not gaining airspeed or altitude fast enough. I saw glimpses of good days with this student; but it wasn't consistent. We were trying to work with them & get them a license; but they weren't understanding the basics- like needing right rudder for takeoff and landing- even at 100 hours & being told every flight they need right rudder. I spent maybe 7 of those hours with them; but was supposed to be co-working with another instructor to help them achieve their goals. Never having been in a situation like that- low power plane & off centerline that much; and as a new instructor; I learned lots of valuable lessons that day that I will carry with me for the rest of my training days.