Is "yeast" a bad word?

Anonymous
I am curious to know because my 8 year old son got detention for using it. During free time in his English class he wanted to explain his classmates how his mom makes bread. He said "you put flour, water, and then yeast," and his teacher was offended by the last part he gave him detention for two days. The principal warned him not to use profanity again.

Since, when has "yeast" become a curse word?
Is "yeast" a bad word?
4 Opinion