A choir member related to me that her High School Choir director wouldn't let the Alto section pronounce any S's. I knew of a choir director who spent as much time teaching the exact pronunciation of each word as he spent teaching the music. These strange situations are the result of directors striving for good choral diction. Diction is what the singer's mouth, tongue, jaw and lips are doing to make different vowel and consonant sounds. Tension draws the lips back, causing a harsh sound that doesn't blend and doesn't harmonize.
Tension also causes a smaller mouth opening, resulting in a smaller sound.
This damages resonance, breathing, and ideal muscle tone. This damages the vowel sounds and the articulator flexibility. is unaware of or indifferent to rhythmic pulse. Don't make a big deal out of diction per se. Place the first vowel of the word on the downbeat. In order to do this, the singer must put a beginning consonant before the beat, but don't tell him per se.