What is a rise in a society's population that is concentrated primarily in major cities

A rise in society's population that is concentrated primarily in major cities is known as Urbanization.

Urbanization may occur both from the population shift from rural to urban residency as well as by population growth within the cities and can be understood as the the gradual increase in the proportion of people living in urban areas as opposed to rural areas.

he sent troops to protect the students as they entered the school.