The headmaster is irritated by the truth Sekhar tells him, because the music he asked about was a replacement for something missing in his life. After Sekhar tells him he doesn't like his music, the headmaster gives it up.
When the headmaster calls Sekhar after their discussion, he's clearly annoyed even though his words are positive. He says:
"Your suggestion was useful. I have paid off the music master. No one would tell me the truth about my music all these days. Why such antics at my age! Thank you. By the way, what about those test papers?"
The headmaster loses something he values when he takes Sekhar's opinion to heart. When he was discussing his love of music and lessons with Sekhar, he said that God hadn't given him children but was merciful enough to give him music. It was a replacement for something he wanted deeply and now he's lost that too.
His annoyance is made even more clear when he asks about the test papers. He wants Sekhar to take care of 100 papers and before their discussion about music, he gives Sekhar ten days to complete them. When he calls that night, he insists Sekhar finish all 100 that night. However, Sekhar thinks to himself that sitting up all night grading is a small price to pay for the luxury of the truth.