They sell pregnancy tests at the dollar store. If you don't see them on the shelves, the cashiers probably keep them behind the counter because they're stolen a lot. Just walk up and ask for one. They'll either cost $1 or $2, depending.
They have a variety of tests at Walmart. The off brand works just as well as the name brand. If you're ever confused, ask someone in the pharmacy to help you.
I've bought tests at Walmart and Target as well as the dollar store. I don't remember ever paying more than $18 for a box of tests, and it was a multipack. There should be an Equate brand test available at Walmart, and it should not be more than $20 with tax.
Or you could walk to Planned Parenthood or take public transportation there, or maybe ask a friend if you can borrow their bike. You can also ask your school nurse about testing options. I know you said they don't have tests there, but he/she can either help you to test or will help you find a place where you can.