Similar to investing, peer-to-peer lending is where you loan money to someone else, and they pay you back with interest. The great thing about peer-to-peer lending is that you can lend as little as $25, and your loan will get bundled into a bigger loan for the borrower. Each month, they'll pay you back principal and interest. You can snowball this initial investment into many investments because you get your principal back as well each month. Many lenders earn over 5%-7% on their investments. Check out Lending Club to get started.
Whether for high school students or adults, you can monetize your expertise by teaching people with less experience or knowledge than you in that subject area. You can work with an established group like Kaplan for, say, SAT tutoring, or you could try hanging out your own shingle and making your services known either to students, parents and schools in your community. If you are targeting adults, you can create your own website or list your services with adult tutoring companies.
You can either organize a yard sale yourself (which is kind of a hassle), or better sell the items through ThredUp, which will pay somewhere around $2-$25 for staff from Gap, Banana Republic, H&M, etc. You can browse around to see how much different items are worth. The best thing about ThredUp is that all you need to do is pack your clothes in a box and then just ship it to them (shipping costs are covered by the company).
Unlike school notes, people need notes taken for everything. For example, senior citizens and disabled persons consistently need help taking notes, and many local Heath and Human Services departments pair up note takers to people in need. If you have time and are willing to travel around town, you could earn a little side income taking notes for the needy.