Jean Piaget -- Educational philosopher or psychologist that practiced during the early to middle 1900's. His theories closely linked children's cognitive developments with their physiological developments. Basically, he said that young children are not capable of the kind of thinking that adults can do, not because they have not learned to do it yet, but because their bodies have not matured enough to allow that kind of thinking. He proposed certain stages of cognitive development as a child gets older. Sensorimotor, preoperational, concrete operational, and formal operational. Piaget's model fits traditional education systems.
Lev Vygotsky -- Educational philosopher or psychologist that practiced during the early to middle 1900's. Unlike Piaget, Vygotsky supported that "learning is a necessary and universal aspect of the process of developing culturally organized, specifically human psychological function." Basically, social learning comes first, then development. Vygotsky's model does fit the notion of having groups of learners together, but would go more in line with a montisorri type education system.
Benjamin Bloom --Educational psychologist who classified educational objectives. His tiered concept of learning is known as Bloom's Taxonomy and moves from basic knowledge and memorization to application, construction, and evaluation of educational topics.