Why did Mary ensure that Kelly's prom dress was conservative when she attended Booker's prom?

As it turned out, both Kelly and GT were going to Booker's prom. Mary was worried about Kelly going since she was so much younger than most of the kids. Even though Kelly was going with a nerdy guy, Mary was afraid that some of the older boys might take advantage of her. Mary made sure that Kelly's dress was conservative which Kelly was less than thrilled about.