If a student is causing trouble in the classroom, giving the infamous "Teacher Stare Down" might remind them that they aren't supposed to be doing what they were just doing.

If it is an object that is distracting the students such as a computer, fidget, or gum then removing it to see if it it helps any would be beneficial. Make sure the student knows they can get it back later though to prevent a temper tantrum!

If it seems like it is a general concensus that the class is not focused on the lesson, then consider getting them up for a minute or two to stretch and them have them take a couple of breaths to help their brains refocus on what they need to be doing.

If there are students that are messing around, try and pick out one student that is not and call them out for it by saying something like "I love how _______ is sitting quietyly waiting for further instruction right now!" In addition, maybe give the small group of students that are ready a piece of candy to reward their behavior.

If it is a consistent group of students who are causing mayhem in the classroom, consider setting up your chair or desk right by them so they will be a little more hesitant to disrupt class.

If all else fails, have a designated safe seat in an isolated area of the classroom without distractions. Make sure that the students know that it is not a punishment to be sent to the Safe Seat but just a regrouping exercise.