Instructor Pilot reported a high CO reading during initial departure climb. Pilot requested and was provided a landing clearance for maintenance support.

Narrative

Upon climb out; a student and myself were given a right downwind departure to the west from ZZZ Runway XXR. Abeam midfield; our Sentry (ADSB-In) device notified us that we had high CO reading 78 ppm. I use a portable CO monitor to help assist in CO detection; and it was showing ranges from 70-90 ppm. I notified Tower that we had high CO in the cockpit and asked for a clearance to land; and full stop and taxi back to the ramp. They replied by asking if we need any assistance; to which I replied 'no assistance needed'. Upon discovery of the abnormality; I took controls from my student; and then configured the aircraft accordingly to help aid in the CO exposure. On short final; roughly 1 mile; my student and myself started feeling the onset symptoms of the CO exposure (slight light headedness) and also started to smell the scent of exhaust fumes. We landed safely; and didn't require any medical attention afterwards. We notified our Maintenance team of the issue; and they will try and resolve.