en versus à

To be in kindergarten can mean two different things in English, which are only becoming clear in the context.

"My boy is in kindergarten," meaning that he is, e.g., old enough and is a kindergarten child. On the other hand, it could also mean that, right now, he's not at home but at the kindergarten right this moment. These two meanings are differentiated in French by which preposition you use:

Mon fils est en école maternelle. -- He might not be there right now but in principle he goes to kindergarten.

Mon fils est à l'école maternelle. -- He is in kindergarten right now.

Bonjour Karen,

I suspect this is a reasonable reason:-

 "Préposition marquant en général la position à l'intérieur d'un espace, d'un temps, d'un état."

My reference is https://dictionnaire.lerobert.com/definition/en.

Bonne journée

Jim