At the time of writing (July 2014), Python is currently the most popular language for teaching introductory computer science courses at top-ranked U.S. departments. Specifically, eight of the top 10 CS departments (80%), and 27 of the top 39 (69%), teach Python in introductory CS0 or CS1 courses.
It narrowly surpassed Java, which has been the dominant introductory teaching language over the past decade. Some schools have fully switched over to Python, while others take a hybrid approach, offering Python in CS0 and keeping Java in CS1. However, at the high school level, Java is still used in the AP (Advanced Placement) curriculum.
Read more at the ACM blog.