I never heard of this movie, but I'd guess it's about Emmett Till. Checking further... Yes, it's about 14-year-old Till who was murdered after allegedly whistling at (or catcalling) a 21-year-old white woman in 1955 Mississippi. Here's the movie trailer.
What is this mother's complaint about the movie?
Same as it would be if they showed "Schindler's List".
You do not show children things that are for mature audiences without having their parent sign off on it first.
I know this tends to confuse a lot of people, but they are not "your kids" and even if they are in your school your power over them is very limited.
Mandatory watching for elementary school children, is my guess.