junior in high school - 11th grade (for kids 16-17 years old)
year book- a book that contains a picture of every person in a particular school
to sign yearbooks- an activity where people in the school write messages and sign their names in other peoples' yearbooks
wrapping things up- in this context "wrapping things up" means "finishing things up"
summer vacation- time during the summer when there is no school
get huge- in this contex "get huge" means to "increase body size" (by exercising and lifting weights)
lacrosse- popular sport in America
to see how something pans out- to see how something happens happens in the future
kickin it- in this context "kickin it" means "relaxing", "hanging out", or "spending time with no plans or serious responsibilities"