Setting firm boundaries is one of the keystones of classroom management. Strong boundaries are complemented by vigilant classroom monitoring and providing students with reasonable, proportionate consequences and incentives.
At the start of any class period, don't start teaching without engaging the students. Thought-provoking questions or statements get students involved quickly. These openers needn't always relate to the main topic of the class. Focusing on disruptive students can help keep classroom atmospheres keyed for learning. When dealing with these difficult cases, try to avoid embarrassing or berating students. Simply standing next to a disruptive student or asking a topical question can end counterproductive behavior quickly. Though disciplinary measures are sometimes required, positive incentives are equally critical.
Staying watchful is a very important for maintaining an appropriate learning environment. When teaching, stay positioned to monitor as much of the class as possible. Avoid turning away from students altogether unless it is absolutely called for. If possible, assign board-writing to students with legible handwriting. Try to position shelving and class supplies so they don't block students from view. When moving around the classroom, use different movement patterns to keep students from memorizing the best times for disruptive behavior. These tactics can stop classroom problems before they start.