I managed to get 8 out of 18. But that's just like guessing. I know, sometimes there is something about a face that let's you instinctively think of one nationality. Once a Japanese friend showed me a picture of other students with whom he took a summer course. I pointed at a girl and said. "She must be Czech." Honestly, I can't say, what I found so Czech about her. It was just a hunch. She could have as well been American, German, Danish or whatever.
I once made an experiment on italki asking other users to guess my heritage, as one user believed that I was Native American (American Indian).
The guesses about my heritage were: East Asian. American Indian, Spanish speaker from Central America and a mix of Spanish and some indigenous tribe, Hawaiian, Alaskan, Filipina, Mongolian, Taiwanese Aborigine, Navajo from Arizona, Chinese, mix between Taiwanese mother and American/British father, Cambodian, Inuit, Peruvian, mix of European and Mongolian/Chinese parents, Japanese. In another thread, someone also came to the conclusion that I could be Iranian. Also, often people think that I'm Kasakh. There seems to me something very Kasakh about me.
So, even though there are sometimes faces that give us hunch, I do believe that in most cases we can't guess the ethnicity correctly just by looking at a face.
La Liseuse, you saw and heard your students in person and I think that's different than just looking at a photo. Body language and voice add a lot to pinning down the ethnicity.