A kind of Bengali

I did my 11th and 12th standard studies in a residential school called DAV Public School in Purulia, West Bengal. One evening I accompanied some of my classmates who went to a shop to buy some stuff for themselves. I didn't have anything to buy. So, I was standing quietly at one corner. An old man mentioned as O below struck up a conversation with me. S is me.

O: So, are you a student?
S: Yes.
O: Which school do you study in?
S: DAV Public School.
O: Ok, what's your name?
S: Susam Pal.
O: Oh, so you are a Bengali.
S: Yes.
O: Which caste do you belong to?
S: I don't know.
O: You don't know? What kind of Bengali are you?