Schools are allowed to set their own policies on legal representation in internal matters (except where stipulated by law). It's only when a dispute hits the legal system that a right to an attorney is sacrosanct.
A reasonable person, lets say a neurologist or developmental psychologist, might very well say there are differing levels of intelligence between girls and boys. It depends on what you're measuring. When they study intelligence closely enough, there are absolutely differences between the sexes. I don't think it's sexist to say that girls are generally better at interpersonal communications than boys (a form of intelligence). Even if it's that debatable (and I'm sure it is), moving from a disagreement on opinions to a title IX complaint is not warranted.