Flight Instructor on training flight with student reported a runway excursion during landing rollout.

Narrative

Start with an uneventful instructional flight for primary student; he's at pre-solo phase. At the practice area; we did simulated engine failure on Vy and Vx climb out at 3000 [ft.] AGL; and steeps turns. Then we went to an uncontrolled airport for 2 landings. The student did great; so we head back to the home airport. Traffic pattern was busy. We had two traffic in front of us; and two traffics behind us. The student had the flight controls. After we touched down; while still in ground roll; Tower called if able exit Taxiway 1. The student turned to the left; but our speed was too fast to make that turn. We already slightly passed the taxiway line. My reaction was to help make the turn instead of taking control and go straight on the runway. That was a bad judgment call on me. When I realized we were not gonna made it; I don't want the airplane to skid and loose control in a turn; so I chose to avoid the lights; and we ended up in the grass; [and] called the tower right away. He called the airplane behind us to go around. Then I used the power to taxi back to the pavement and taxi back to park. Called the Tower afterwards; he said next time I should tell him unable; don't try to force it. Ironically; 'unable' did pop out my head when I think we were too fast to make the turn; but I was incompetent to say it and execute it. I am very embarrassed because I thought I teach all my students to not force the turns; but I didn't do it today. It's 100 percent my fault; I am not a good instructor.