Question

Where's Sudha?She's..... school. (Choose the suitable preposition )

Answer 1

The preposition to be used is ‘at’.

‘in’ should only be used in cases where you are physically inside something, or actively doing it.

That doesn't count for locations, because you aren't inside the school, because the school is made of brick and mortar. You are standing in the area surrounded by the school.

“At school” is purely physical location. The question asked here is “Where...?” so obviously Sudha’s location is being asked for.