Don't believe me? Read this study by R. Allday and K. Pakurar. Greetings students individually increased students' on-task behaviors. Another study by S. Patterson confirmed similar results (sorry, a free full-text article isn't available for this one on the web; check the e-databases at your school for the full-text article using this abstract).
Bottom line: Greet students personably as they enter the classroom and see the difference in their on-task behavior.
Just wondering....Do you think this would work at my house? How about at the gym?
Try it the first week of school - and all the other weeks thereafter.....