None of my business, but maybe you should focus on what's causing you to fail in school first. I get that it's the summer, now, and there's no classes, but if there's certain subjects you're falling behind in, there's resources online to get caught up in those subjects before the fall. What's your situation? Maybe I can offer some advice.
You could hold a yard sale and try to sell your stuff, but you should talk to your parents first, because... they might not appreciate that.
Depending on how old you are, there might be businesses offering part time work over the summer. What the age limit for that is, and what additional requirements there are, will vary based on where you live.
Do you have any potentially marketable skills? For example, web development for small businesses is something you could potentially offer that would pay better than yard work and generally be less tiring.