New school..drug test..need help

ok i have heard so many different time periods that you have to pass a drug test. i know that it just depends on the person. but ive heard everything from 7 days to 45 days. and i just dont know.
the new school im going to has a drug test at the beginning of the school year. it is a piss test. im looking at about 20 days clean till the first day of school.
im 6'3 185 16 years old and work out every other day. not to mention i drink a lot of beer and piss a lot.
since about april till june 20th. i smoked dro(yes dro no bullshit) just about everyday or every other day.
now im worrying about this drug test. should i be worrying?
and how long do you think it would take for me to be clean.

sorry guys but im looking for no bullshit answers please
people who know what their talking about or people who have had experiences with this.
please im sick of hearing bullshit from people who THINK they know what their talking about.