Flight Instructor reported they scheduled a flight in an aircraft grounded for maintenance and did notice a missing flap during preflight. Once airborne the Instructor noticed irregular aircraft handling and returned to the airport and landed safely.

Narrative

The flight was originally scheduled for a flight lesson in a different aircraft. That aircraft still had an open dispatch from the night before and a missing can/aircraft discrepancy log. This put us behind schedule on the timeline to complete the lesson. This is when the external factors started to build. That is when I scheduled the lesson in another available aircraft that was available on our scheduling system; along with the keys/aircraft discrepancy log. I was not aware that the aircraft was grounded for maintenance. We conducted a preflight on the aircraft and we did not catch the missing flap. We then completed all checklist and performed a few landings in the pattern. I noticed an irregular flight condition and I decided to cancel the flight to investigate the issue. After landing and parking I noticed the missing left flap. No damage occurred to the aircraft and no bodily injury occurred to I or the passenger. I then proceeded to secure the aircraft and return the aircraft discrepancy log/keys to the maintenance office. Human Factors: Having to reschedule to a different aircraft with full availability; indicating no discrepancy. Being behind schedule on the original timeline to complete the lesson. Being a low time instructor trying to complete a task for a new student.