Can I get syphilis? Please Help?

Ok im 15 and my girlfriend is 15 and my teacher told me Syphillis is a really common STD and I know that STD's are usually transmitted when you get infected from other people but im pretty sure my girlfriend hasnt had sex with anyone else before actually I know for a fact she hasnt. But just to be sure can some one explain to me more how Syphilis works and how quickly it is to catch Thanks