Collective nouns

I HAVE a problem with pronouns. In your column on Jan 6, you wrote: “A herd of cows that belong to the farmer are grazing in his field.”

According to my English tuition teacher, “a herd of cattle is”, because there is only ONE herd of cattle. Other examples include: A bunch of bananas does not cost much. However, we say a group or a number of students are ... .