It seems from your comment on Ronald Soles's post that your intended question is not about the specific subject being taken (i.e. French class or Math class), but about the level of schooling you are in, corresponding to how old you are and how long you have been in school.
A quick Google search has confirmed that the Indian school system refers to its levels of schooling as "classes", meaning one can be in 2nd class, 6th class, 8th class, etc.
In Canada and the USA, we call these grades, not classes. Here, a "class" refers to the subject you are taking, like math or science or history. In Canada/USA, you start school at age 3 or 4 in what is called kindergarten or primary school. Then at age 6 you enter "grade 1" or "first grade", and each subsequent year after that you move up a grade level until grade 12 ("12th grade"), which is the last year before university. "7th grade" or "grade 7" corresponds to the year of school when a student is 11-12 years old. This is therefore the same as "class 7" in India.
Additionally, in the UK they call these "years" or "forms", as in "Year 7" or "6th form". So as you can see, there are many different words for one's current educational level depending on which country you are talking about. Regardless, the format of the English phrasing is the same.
So, the way to ask this question would be:
"Which grade are you in?" or "What grade are you in?"
of which, the second is more common and idiomatic than the first, and to which the answer would be
"I am in 7th grade."
So, if you are talking about your educational class level in India, you would say "What class are you in?"
You could also say, "In which grade/class/year are you?", which is technically grammatically correct but not the common way to phrase it.