"She is in school " = accepted now.
I see the following answers being suggested as well :
"She is at the school"
"She is in the school"
My English teacher taught me that the article was not needed.
The dictionaries that I use, also don't use the definite article.
My question is :
Is it grammatically correct to use the definite article here ?
If yes, what is the difference in meaning when using/omitting the definite article ?