class. classroom. / in class, in the class.

 

She is in class = She is currently taking part in a lesson.
She is in the classroom = Simply a statement of her location.

https://www.usingenglish.com

 

Grammer, if you have a look at the dictionary entry linked to by Wilma, then you will see:
In class (=during the class)
​
and
a) a group of students who are taught together [↪ classmate] in a class
We're in the same class for math.
​
Plus, the mention made in this thread about class meaning classroom in American variation of English, that is:
in the class - in the classroom

I think the dictionary source and some of the comments you got here are reliable enough so that you could use the expressions correctly. :)

 

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