There are several good reasons.
First, the school needs to be able to account for the students.
Second, they need to release the students to the parents/guardians and not release them to the wrong people. That would make a bad situation worse.
Third, most of these students live miles from the high school. They cannot just walk home. And you sure don't want students walking around, and parents looking for them at the schools. That would be mass confusion.
Fourth, highway 285 is the main route, and, IIRC, the only paved road and it is blocked off. The situation was wild enough without having hundreds of distraught parents driving around.