Around what percentage of universities require(d) Ancient Greek and Latin for admissions?
I don't think any "major" or "top tier" universities in the USA have any language requirements other than English. Even then, you don't really have to speak English as much as be able to manage HW's and exams in English.

St. Johns would certainly appreciate it but most courses won't read classics in their native language. 

My friend is finishing a PhD in classical history at an Ivy League program and he was unique in his knowledge of Latin from high school. I'm pretty sure 0 colleges require Latin or Greek.