As you know, Eton College is not a university. It's a private (what they term "public" in the UK) prep school. High school. Eton is right to keep girls out of there. Girls are a fatal distraction to lads during the intense learning years, and vice-versa.
Betsy came across a sample of their entry exam for 12 year-olds. I think it's better than the SAT: Eton College King’s Scholarship Examination 2011.
Basically, they are looking for IQ, curiosity, and capacity for the life of the mind because they will want to push you beyond your limits if they accept you.