If your professor has an ounce of integrity, he will look at the evidence and announce to the whole class that he was mistaken.

I've just spot-checked some books from Project Gutenberg. In the works of Mark Twain, "impact" is used once as a verb and twice as a noun (e.g., "Now you see that this constant impact of crime upon crime protects you against further commission of crime." - "Theoretical Morals"). In "When the Sleeper Wakes", by H. G. Wells, it's used four times, always as a noun ("He felt something soft against his extended hand, and the impact of a broken fall quivering through his arm. . .")

For more modern examples, enter "impact site:www.time.com" in Google to find how the word is used in Time magazine. When I looked at the first ten pages returned, every single use of "impact" was a noun.

What surprised me was that, according to M-W's Collegiate Dictionary, the verb use of "impact" goes back to 1601, while the noun use came along as late as 1781.