I would also add that slow flying is not something I would dedicate a whole lesson on; it is something that can be tagged onto the end of other lessons, ie: if you have a 10 or 15 minute flight back to the airfield at the end of the exercise I would normally use some of the return leg to the airfield to practice slow flight.
As said I view slow flying as an exercise to refine handling and extension of the effects of controls rather than a pure stall awareness exercise.
80 knots standard speed for a PA28, your school is using too low a power setting, a PA28 will happily cruise at 90 to 100 knots.