Impress a Guy in High School

There are a lot of girls who have trouble with finding how to approach a guy (or guys approaching girls) but it's all about having confidence.

Steps

Have confidence. It's one of the top rated things that impresses guys. If you like who you are, chances are, he's going to like you too.
Smile. It's the most simple, yet best thing you can do to impress a guy. They appreciate a smile. Make sure you smile whenever he makes eye contact (don't stare for too long though, he'll be freaked out).
Learn about his interests. Find out what he's into whether it be sport, reading or whatever and show your interest. He'll be impressed that you like him enough to ask about what he likes.
Compliment him. If you compliment a sophomore guy, he'll be more impressed by you; guys like to feel confident. and He'll be more likely to come and chat to you.
Be Yourself Probably the most important thing here is to just be you. He'll be impressed by a girl who's independent and can just act her usual self whoever she's with.
Dress Well The first thing he's going to notice is what you're wearing so dress to impress. A little light perfume won't hurt either - he's going to like you a little more if you smell good.
Wear light makeup to enhance your facial features, but not too much. Over doing it may sway his opinion about you in a bad way.

Don't act too nervous or excited when you talk to him - he might become arrogant knowing that he has this 'effect' on you.
Don't try too hard - he won't want someone who's clingy and desperate.
Don't ask your friends to ask him out for you, or tell them to tell him you like him - it's kinda immature and he won't appreciate it as much as he would if you just told him yourself.
Just be cool, casual and be yourself.
Don't stalk him.