Because what you do in high school is going to determine your future.
"Junior high school wasn't important, elementary school wasn't important, surely high school won't be important!"
As a teenager (I am one now, so believe me, these things go through my mind too), you're more focused on earning money, meeting your sweetheart, and driving the "coolest" car out of any teenager in town. You dream of owning a Ferrari when you're 40 years old. You want to retire young as a multi-millionaire. You want to get married and have a few kids. You want to be famous. You want to be the President of a business you started and earn loads of cash. You want your kids to say about you, "I have the coolest dad/mom ever!"
Do you want all this to come true? You need to try hard in high school. Your high school grades depend on what college you go to, and how you do in college will determine what you do for the rest of your life.
If you don't try your best and succeed in high school, you won't get into a good college...and you won't get that Ferrari you want, or retire young as a multi-millionaire!
Your future actually starts now as a teenager. You may not realize it since you're still young, but how you do academically when you're 17 will determine what you're doing in 30 years from now - whether it's working in a supermarket as a cashier or being retired in Florida.
You are now walking down the road to determine your future, so the time to work hard is now.