Right And Wrong

Teacher writes a sum,
3 + 7 = 9, on the blackboard.

Teacher: Is the sum right?

First Student: Wrong.

Second Student: Right.

First Student: Wrong.

Second Student: Right

First Student: 3 + 7 should be 10 and not 9, right?

Second Student: Right.

First Student: Then why did you say 3 + 7 is 9 right?

Second Student: Because you say it is wrong and I agreed with you.