Flaps do not slow down a plane, that is not their function. Flaps increase the area of the wing allowing it to produce more lift. Being able to produce more lift allows the plane to fly at a slower speed and still remain airborne.

Trainers really don't need flaps as they are designed to be able to fly at slow speeds without using the flaps. Also, it just adds another layer of complexity for a student to worry about. When a student is training the goal is to KISS.

Ken