Bone marrow. The inside of your bones are filled with a soft tissue called marrow. There are two types of bone marrow: red and yellow. Red bone marrow is where all new red blood cells, white blood cells, and platelets are made.
Mine is 8m x 7m (3m is 10 feet, so around 27 x 23 feet). This is normal for an Australian classroom, although there's a fair bit of variation.
Skills such as effective classroom management are central to teaching and require common sense, consistency, an often undervalued teacher behavior, a sense of fairness, and courage. These skills also require that teachers understand in more than one way the psychological and developmental levels of their students.