I asked the students to organize themselves from ugliest to prettiest
I cannot believe how mean you are. Why would you do this?
I thought that it would be interesting to see how they would rank themselves
Your little experiment while interesting, is completely immoral and disgusting to these students. You should be a loving and caring teacher, not judgmental and rude. Doing this to your students only makes them hate you and each other, and show the ugly judgmental side of all of them.
I don't care, I just think it is an interesting experiment and I don't think I'm not hurting anyone.
You might not even realize that you're hurting people. Some might have big issues with the way they look and by conducting your experiment you might make them feel even worse.