i totally agree with slipknotdude, you have to tell the truth to your familly and show them that smoking weed doesnt afect you as a person or as a student..... slowly they are going to realize that you are right weed isnt bad as they think and maybe one day you all will smoke a big fat "peta" as a beatifull and happy familly....
good luck.........................( i am from south america so i speak spanish, sorry if i dont express my self as i wnted to)