Also, if you've got some clothes that you've outgrown that are still in good repair, you might ask your folks if you could consign them. Basically, you clean and iron them and take them to a consignment store and they give you some of the money they get from selling the clothes. Likewise, there are children's consignment shops that sometimes take toays and such in addition to clothes.
If you want to do the yardsale thing, but don't have enough to do one yourself, you might ask your friends and neighbors to see if anyone you know is planning one for the future. Then you could take your stuff to their yardsale.
You might also consider something you could sell at school. When I was in high school, there was a guy that made a tidy profit selling t-shirts one year to the kids in the Advanced Placement classes. On the front they said "AP Hell" and on the back "No Sleep, No Pain, No Problem". He got price quotes from the local t-shirt shops ahead of time & had one sample printed up, marked up the price $4-5 per shirt, and took orders. He wrote receipts for each order, used the money from the orders to purchase the shirts and delivered the shirts. They weren't more than $15, so were affordable for most students. I think he ended up selling around 200 shirts, so made about $1000 in six or seven weeks. It was a lot of work, though. And you'd want to get approval from your school before you put much money or time into something like this.
Best of Luck,
EthansMom