Flight Instructor reported a loss of aircraft control during landing which resulted in a runway excursion.

Narrative

I was involved in an incident where the aircraft accidentally clipped the grass. As I was inbound to land and make a full stop at ZZZ Airport; I was briefing my student and letting them make the calls. As an instructor; I ensured everything was complete; including the landing checklist and properly receiving clearance from the Tower to land on Runway XXR. Approaching the landing; everything was fine until the point where we were exiting the runway. While exiting XXR onto Taxiway 1; my student was applying left rudder; and I was showing him how to gradually brake. Unfortunately; I had to apply more pressure on the brakes to slow down; but my seat slid back; causing my feet to lose contact with the brakes and rudders. The Skyhawk brakes were mushy; requiring more pressure than usual. After accidentally taxiing onto the grass; I checked to ensure my student was fine. Thankfully; the aircraft was stable; and there were no damages to the taxiway or grass. After leaving the grass; I immediately returned to Taxiway 1 and called Ground to inform them that we were ready to taxi back to the ramp. Unfortunately; this situation was avoidable; and as an instructor; it's crucial to ensure safety. I could have attempted to taxi to the end of the runway if possible. As the main cause of this situation; I need to ensure that my seat can lock and; if not; get that sorted with the maintenance team. Being shorter makes it challenging to apply full brakes while the seat is sliding back. In conclusion; to prevent this situation from occurring again; it's essential for the seat to be completely locked and positioned upright.