* Definition of "school": The term "school" can refer to elementary schools, middle schools, high schools, or even universities. The largest in one category will be vastly different from the largest in another.
* Data collection: Comprehensive, publicly available data on student enrollment for every school in the US isn't readily compiled in a single, easily accessible database.
To find the largest school in a *specific* category (e.g., the largest high school), you'd need to research enrollment figures for schools in that category within specific states or districts.