Your professor is kind of an idiot because he somehow manages to understand white privilege, white washing, and how racism affects poc etc, but FAILS to accept that it cannot be the responsibility of only people of color to educate white people. Imagine if we lived in a world where his logic was applied, and only the oppressed were allowed to speak of their oppression.
When people of color are expected to educate white people as to their humanity, when women are expected to educate men, lesbians and gay men are expected to educate the heterosexual world, the oppressors maintain their position and evade their responsibility for their own actions.
Also, if your class desires a representative of another race to tell them how movie castings are white-washed simply because they “understand what it’s like” point them in literally any direction, seriously. There is nothing a white person can say about racism that a person of color hasn’t already said, written books about, or protested in the past. But the problems still exist, why? Because people of color are not in a position of power and will likely be ignored, talked over, or silenced when we speak out. The only way to really end racism is not if all people of color speak out about it, it’s if WHITE PEOPLE LISTEN AND EDUCATE EACH OTHER. I applaud you for taking responsibility and using your privileged position to try and make others aware.
You are self aware and self critical, and you are trying to teach others to do the same by example. If your class opposes this crazy notion that a white person should defend another race, that in itself is a symptom of white supremacy. You never claimed to understand what we feel, you simply listened and understand that it’s disgusting when hollywood cast white actors to play historically black roles.
If it helps you in the future to use this ask to support your argument, by all means sister.