Im guessing walking because you walk between classes.
The respiratory system mainly consists of the lungs. The lungs give oxygen to the body, when you inhale air. They are responsible for removing carbon dioxide from the body. The circulatory system mainly consists of the heart. The heart pumps blood through the body, giving us oxygen. The heart is responsible for pumping blood and creating new blood cells. If we did not have a heart then there would be no blood, therefore, no oxygen. If you think about it, the body's oxygen transport system takes oxygen to the working muscles, through the circulatory and respiratory systems working together. The main workers are the heart, blood and lungs. The whole oxygen transport system works in a cycle.
You must consider if you are putting yourself in danger by assisting them and the information you should provide first is where you are located what you seen happen you should ask them if you should do anything to help ?
The answer is endocrine system because it deals with hormones around the body and is controlled by the pituitary gland.