Yes, you must have a valid passport to go on a high school exchange whether you go for 3 months or 10 months. When you travel to the US, you must have a biometric passport (i.e. with a data chip). The passport must also be valid for at least 6 months after you return home. If your passport expires before then, you should get a new one as soon as possible. Also make sure that your passport is not damaged in any way and that it does not have any missing pages. Repairs with tape and glue are of no use – you will need a new passport.