The more or less exact time period needed for school management software development can be estimated only when you already have at least some general requirements. The most important factors that define the time needed for the project realization are its complexity, the number of features to be built, their type, the required integrations, the chosen tech stack, and the number of team members.
On average, the creation of such a solution can take from 6 to 12 months. However, if you have a more complex project, it may be required to invest more time in its development.