Yeah it's a highschool class, and here at my Uni they assumed you know nothing and started from the ground up. I'd suggest Java, since that's probably what your class will cover. However I like Python a whole lot more.
Don't stress about it, but it'll undoubtedly help if you learn whatever you can over the summer. You'll likely do just fine if you learn how to make variables, initialize them, basic input/output, control structures like if else and loops, and learn how to write functions with parameters.
But like some other dude said if you were told to know something before hand then it's a different story.