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.
Modular portable classroom can also be leased or purchased, although purchasing is more economical for long-term use. They sell for $40 to $200 per square foot, depending largely on size and the amount of customization you require. Total costs range from $50,000 to $200,000.