IMO, your teacher shouldn't be asking you to debate what anything "is" a symbol of. Symbology is inherently subjective. There may be (idk; I believe there are, but I haven't spent enough time in the South to test it) a lot of people who display the confederate flag to show that they are "of the South", or that they support state's rights and a limited federal government, or similar, and have no racist intention. And this wouldn't make it any less true that to a lot of people detest it as a symbol of slavery and racism. Or that there are probably some sick people who embrace it as a symbol of racism.
I can tell you what a symbol means to me, explaining why I display it. You can tell me what it means to you, explaining why you object to my displaying it. We are both right about what it means, as neither of us is saying "what the symbol means", just what it means to each of us.
I think, if I were debating this, that I would just stick to the argument that symbology is subjective. Because I doubt you can "disprove" that the flag is a symbol of hate if you accept that there are "objective" meanings to symbols.
But you can talk about corruption of symbols in that argument. The swastika, which is now a symbol that means racial hatred and white superiority, used to be a sign for the sun. And the mirror image of the swastika, though it obviously looks very much like the swastika, is a luck symbol in some East Asians cultures. No objective meaning.
And for a slightly less touchy example, you could use the sign of the Deathly Hallows in Harry Potter. If you're into that. I'm sure there are other literary examples as well, but that one comes to mind now. Krum gets angry and almost challenges Mr. Lovegood to a duel because he associates the sign with the dark wizard Grindelwald, when the sign "actually" represents the Deathly Hallows. Hope this is helpful.