Is taking Adderall unprescribed a bad idea?

If by bad idea, you mean illegal, then yes. If you are a juvenile, please do not be stupid. It doesn't help your memory at all, but it will help you focus if you do indeed have ADD. If you do not, someone will notice that your behavior is not quite right and start to wonder if you are on drugs. Which you will be, so just study and get rest. If the diagnosis does come in as ADD, you will get a prescription and will likely be allowed to retake your midterms.