There is no consensus on that, and it's not just about teachers. It depends on the mathematical field and tradition. It usually starts at 1 in German, at 0 in F

There is no consensus on that, and it's not just about teachers. It depends on the mathematical field and tradition. It usually starts at 1 in German, at 0 in French due to the influence of the Bourbakis, and in English I think it's more field-dependent. The original formulation of Peano started at 1.