Clothing should be cheap, but not stained or holed. Knock-offs are ideal. Look for "Boxford" or "Izag" shirts. While being dressed in a gym uniform is a pesky equalizer, create ways to look a cut below in the socks and shoes department. Get knee high sports socks that don't stay up. Shoes that have stepped in way too much dogshit. Borrowed sweatpants and sweatshirts are a solid choice. Your dad's closet, provided he's still around and his wardrobe matches his shitty-paying job at the hardware store, can be a treasure trove.
It's probably okay if you have halitosis and a little dandruff, but don't smell bad. Believe it or not, that's a deal-breaker.
Show the girl some skill you have. Soccer is good because it's a team sport and implies that you get along with others. Karate or kickboxing, God help you, shouldn't be selling points. Guys who know martial arts are not socially adept, hence they learn martial arts. If that's actually your thing, don't lead with that. Basketball, surfing, street-smarts, even being a loveable pervert are better alternatives. Especially the pervert. It's clean and honest.
On the first date it's important to set the standards low. Meet her parents so she can see their disapproval and then rebel against it. Let them assume you're a slacker who is poor out of choice. You
Inevitably, the rich jealous ex-boyfriend will make a move. Don't play it cool. It's only stalking if she catches you checking up on her. And she will catch you, so have a self-deprecating event lined up to glean just enough sympathy so she'll consider giving you another chance. If you can't have a waiter from the country club accidentally crash into you, spilling rich person food all over your cheap clothes, or flaunt a cigarette burn your dad gave you for spilling paint in the garage, then let yourself get constantly flagrantly fouled in the high school basketball game.
Also, don't worry yourself about looking bad next to the ex. He's going to be so unlikeable with his unnecessary possessions and white privilege (that's right - I said it!), that no one's even going to question the morality of holding this 17 year old guy responsible for how douchy he is, despite the fact that his parents have no doubt played a huge role in coddling and spoiling him for his entire fucking life. You could murder a doe-eyed, dimpled child and still not be as hated as this guy. That's not a theory to test but it is the truth. Even if you're the "bad boy," your brand of unscrupulous behavior is on a different level than his and more excusable. It's totally okay for you to be a product of your environment.
However, all of this is completely useless should the girl be from the wrong side of the tracks. It's rare, but it happens. Her self-loathing over her chronically unemployed and unemployable father makes her an easy target, more so if she has the devil's red hair. Any attention you show her will be so appreciated but it requires finesse, lest you become the asshole rich-guy who needs a level-headed dork to set you straight.
Eventually, there's going to be a moment of truth for the two of you, or more accurately, a shit-or-get off-the-pot moment for her. She either accepts the fact that you're a loser but you're her loser, or she snaps out of it and doesn't even acknowledge you in the hall at school. If she indeed succumbs to your indigent lifestyle, you're going to have to at least get a job, maybe make an ill-fated attempt to be in the school play. Baby-blue, ruffled tuxedos for prom aren't cheap and the honeymoon period won't last forever. If she drops you then you can at least tell all your friends you were doing it with her, as they laugh their assess off as she walks by. You should always have a quirky gal-pal you didn't appreciate at first but is waiting for you when you're done riding this 90-minute roller-coaster and just need to feel loved.