Is this inappropriate for school??

My little brother comes home from school and tells me that his teacher told him to either cover that shirt up or go to the office. It’s literally a shirt about “farts”... She got onto him in front of the whole class. My brother is a shy kid and never gets in trouble. He was a little upset about this. He was so excited to show his friends the shirt. In my opinion, this shirt is no way, at all inappropriate. But she was a older teacher about in her 60s and to her, its probably inappropriate.