In a time-limited acute health crisis, an IC might choose to shelter these populations in place because:

Facilities that house minors have a specific need because they are required to retain custody of the minors in their care until released in to the custody of their parents or legal guardians. Due to this special need, Incident Commanders may choose to shelter these young people in place rather than attempting to evacuate them.
Office buildings and high-rise residential buildings generally have lower air exchange rates than single-story residential construction. This means that these large-population structures are better suited for sheltering in place. This fact, along with the knowledge that evacuation routes and resources will be overwhelmed by a large-scale evacuation, might cause Incident Commanders to order populations in these structures to SIP rather than evacuate.