You look great! As far as whether or not the dress is appropriate, I'm not sure. I don't know where you're from, but here in the US you wear Caps and Gowns at graduation that cover up whatever your wearing.
What Girls Said 1
My only remark is that it might be too short for graduation. I know that at my high school, our dresses had to be knee-length or longer. And your's is mid-thigh.
Your stomach does bulge out slightly from the side. My advice is to not eat a huge meal or drink too much water before your ceremony.
Graduating high school is a great achievement, congratulations!