I agree with Farrah, and would do everything that she says, with a bit of spice. The spice being for those most challenging boys/kids.
WIth them you need to just say, "I'd like you to practice with me at recess." Once recess comes you have them practice the rule correctly for 1 minute, silently. They may start goofing off or telling you that they aren't going to do it, so then you tell them this, "You can do it one minute my way, or 4 minutes your way!"
You may want to peruse the ebook called
Industrial Strength Whole Brain Teaching or
Teaching Challenging Teens and look in the section Rebel Clicques. What you have is a situation where you're the new gal coming in on their turf. You're going to have to break them down one by one,

starting with the Alpha Dog.
Keep us posted right here on this great forum!
Power to the Teacher!
Chris R.