Because the first teachers of a student is their parents. If the parents can't speak a language, it means that the student also don't know that language because they learn from their parents. So, it creates a problem when students learn in a language that their parents don't speak.
The prefix in the word 'impartial', meaning 'unbiased', is im-. Without that prefix, partial means biased. Im- is a prefix that gives a word its opposite meaning.
