C150 Flight Instructor reported on departure experiencing a low engine RPM resulting in flight towards terrain; the Flight Instructor was able to return to the departure airport where they landed safely.

Narrative

I was instructor pilot in right seat. Learner; commercial-instrument pilot; was in the left seat. Plan was to takeoff and fly southwest from departure airport 10-15 NM for spin training toward learner's initial CFI prep. Since learner was unfamiliar with this airplane and the airport (bit of a tricky airport for the un-initiated); we agreed that I would do the takeoff and landing while he did the rest of the flying (until we got to the practice area).We thoroughly briefed the takeoff procedure including anticipated liftoff point on the runway and monitored engine RPM; oil pressure; and airspeed-alive. Engine RPM was expected to be 2;350 RPM minimum static. The round (analog) tach was extremely labile; making it difficult to determine what the RPM actually was; so I monitored perceived acceleration carefully and noted that we lifted off where expected on the runway; using the hangars off the side as a visual reference.Once airborne; it was clear to both of us that the engine was not performing well. RPM was only around 2;350 while we were flying at 70 knots. It should have been at least 100 RPM higher than that; from my experience in this type of aircraft and this airplane in particular (I have hundreds of hours of instructing time in this exact airplane). However; I judged that it would be more safe to continue the takeoff than to try to put it back down on the remaining runway. We would not have been able to stop before rolling off the end and down onto a busy road.The airplane was not climbing well enough to clear the tall trees off the end of the runway; so I did what our standard procedure is in cases like this which is to angle to the left of the tall trees and fly over some smaller trees; then scoot back to the right to intercept the straight-out departure leg north of the tall trees.The climb being so anemic; I kept a continuous watch for off-field landing sites; should I need one. When we were at a safe altitude (high enough and close enough to the runway in case the engine died); I tested carburetor heat. It worked as expected but did not improve performance either while on or after I turned it back off.She climbed well enough to get us up to 900 MSL on downwind (TPA is 1200 MSL); after which I landed on the same runway from which we departed without further incident. Taxied in; took the airplane off the line; and reported the issue to maintenance.