Yard care (mowing, weeding) for elderly. Babysitting for neighbors (I would recommend Red Cross course first). Pet walk / feeding for neighbor. Paper delivery. Relative that has a retail business could give you a few days work. Yard sale is possible if you live in where you can do that.
If this is for something that is important (e.g. school trip, prom dress) as opposed to something you just want (e.g. newest cell phone) try borrowing from parents or relatives. As part of convincing them that you are serious, apply - and get - a job to make sure they get paid back. Both of my kids had jobs by the time they were 16. They didn't earn a lot, but it gave them their own money. And it was a stream (OK trickle) of money that would continue. And gave them something for their resumes for college and / or later jobs. And it also gave them bargaining power for future loans.
Well back when I went to school this kid In my class would go to like a sam's club or stores that you can buy in bulk. He would buy about $50 worth of candy bars bulk cuz for 50 bucks you can get about 125 candy bars. Take them to school and sell them or sell them to friends for around $2-$2.50 and you can make a $250 profit off $50 worth of candy.
sell your hair ... if it is virgin there are places that would pay more or less $100. garage sale is another thing i can think of and you have to have a lot of junk and a public area. if there was any quick/legal ways to make money everyone would be doing it. getting a job takes a while but the money is more consistent.
Nowadays, there are various ways to earn cash fast. You can start at your own home by creating some arts and crafts or by baking some goodies. Simple things like that can become great strategies when starting a small business that will allow you to enjoy while earning some money.
I trade video games and make a killing. Try trading in something cheap and it comes in large quantities like cards or even coke bottles.