As 'class' is a noun and nouns always take some determiner. So I would say 'in a class', in the class, in my class, in your class, in her class but never 'in class'.
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anonymousin class
in the class
Examples.
in class
They didn't talk to me that much in class but out of class they'd confide in me.
('in class' is simply a location. It doesn't matter which class it is.)
Many are also in class every other Saturday and for three weeks over the summer.
('in clas