How can one deal with lack of discipline in the classroom?

There are limits to what teachers can do to maintain discipline in a classroom. Many of the factors that affect classroom behavior, including home environment, class size, and even student personalities, are beyond the control of the teacher. 

The first thing you need to do is set clear boundaries and guidelines. Often children (especially teens), as part of their own attempts to build independent identities, will push the limits of any rules or conventions; making rule systems transparent, fair, and consistently enforced actually gives the students a sort of stability that they find reassuring. This means that all classroom rules should be simple, clear, explained to students in terms they can understand, and enforced. 

Next, students with disabilities or behavioral problems should be referred to the appropriate staff to get the treatment they need. Accommodation plans for special needs students will reduce the likelihood that they will become discipline problems. 

Rules and punishments for infractions should conform to all relevant school and departmental guidelines. The better the support you have from upper administration, the more effectively you will be able to maintain discipline through methods such as reporting, detention, suspension, and parental involvement.

As a teacher, you should remain calm and professional at all times, creating an orderly and focused classroom that engages student interest. When students do push boundaries, avoid arguing with them or losing your temper, but remain calm, cheerful and firm. Enlist your students to work with you to create a positive classroom environment. Be aware of what is going on in every part of your classroom so that you can deal with potential problems before they escalate. Praise and positively reward good behavior.