Why are soft drinks banned from many schools?

Not healthy. Thankfully some school administrators focus on the health of the children in their charge and focus on the health and well-being and nutritional status of the students.
Empty calories. Many feel that sugary cola drinks, highly appealing to youngsters, contribute to obesity and improper dietary habits in this age group. They feel a better approach is to provide more healthy drinks and prevent temptation by banning these drinks from school situations.