Kannada teacher

This is my nephew's experience. He is in 3rd standard ICSE syllabus. Last year he was introduced to Kannada in school. But Kannada being his mother tongue, he speaks good Kannada since the time he started speaking. His course includes recognising animals in Kannada. In one of the recent tests, he was given a picture of a cow and was asked to write what it is. The answer he wrote was 'Hasu'. But the teacher had put a cross against the answer and wrote in red 'Dhana'.
My sister went to the school to talk to the teacher, as this was not the first goofup she had made. She argued that 'Hasu' was not a Kannada word.
How can such people teach Kannada?