People are shallow and will give pretty girls higher grades. Effect disappears if you make them take the same class online.
Similar topic teachers in germany/switzerland/austria rated the (identical) answer for a physic problem lower if it supposedly came from a woman, interestingly that mostly disappeared for teachers with 10+ years experience. http://arstechnica.com/science/2016/01/give-teachers-a-physics-test-from-a-girl-and-theyll-give-it-worse-grades/
I think it would probably be a good idea to grade tests anonymously. Of course that does not work if participation or presentations are graded. (Also recognizable hand writing is a problem.)