Which ones are the hottest stars? Coolest stars?

Ok so my teacher said that the hottest stars are those that are red. But I thought the red stars are the coolest? My teacher even said that they are the youngest ones too. And the coolest are the blue stars. OMG! This is so confusing! What's right? My teacher or the book? My book says that the hottest are the blue stars and the coolest are the red stars. I know, I did tell my teacher that he was wrong but he thinks he's right all the time. He even scolded at me for interrupting our discussion. So is he right or wrong?