Going to Germany?

I am thinking about going to Germany as an exchange student either in my Junior or Senior year! My mom wants me to wait till I'm a senior because she says when your a Junior you do a lot of things (such as psats...etc) that get ready for college! So i was wondering what you all thought what year would be better! and what i would miss if i went my junior year or what i would miss if i went my senior year!