Step 1: Read the situation.
A group of students enters the class and behaves violently. We must choose the best first reaction of the teacher.
Step 2: The teaching idea behind it.
Good teaching psychology says we should first understand why a behaviour happens before we react to it.
Step 3: Why students may act so.
Violent or rough behaviour often comes from stress, anger, peer pressure, family trouble, or study problems. There is usually a hidden cause.
Step 4: Check the quick reactions.
Reporting to the head master, calling parents, or turning other teachers against them are all responses that punish or escalate without knowing the cause.
Step 5: Choose the wise response.
The best first step is to find out the reason for such behaviour. Once the cause is known, the right help can be given.
Step 6: Final answer.
The teacher should identify the reason for such a behaviour.
\[ \boxed{\text{Identify the reason for such a behaviour.}} \]