I was in choir my entire run of schooling and facial expression only mattered as far as "Smile during performances". Not once did anyone in my classes get marked down for not smiling enough during Rehearsals. If the OP had been given some kind of warning, I would assume the issue is they didn't smile hard enough during concerts, but since they didn't get a warning it has to be an in class issue in which case the instructor is just being idiotic. Choir is not drama class, you are not there to practice being dramatic, the only facial expression needed is the ability to smile during concerts when not actively singing. If you try to smile the whole time you are singing you will likely screw up what you are singing because some notes and words actively require your mouth leaving the smile formation. Also, some people physically don't have a wide range of expressions they can make. So to mark someone off for not having "enough facial expression" may actually be marking them down for something they flat out were physically incapable of doing. It's the reason that some people, no matter how hard they try or how much practice they put in, will never be good actors. Their faces just don't have the range required to sell the emotions they are supposed to be expressing. Plus on top of that, Choral singers are supposed to be uniform. The only ones who should stand out at all are the soloists, so it is actually better if the people in your choir aren't all trying to do different ranges of expression on their faces. They should all more or less match each other expression wise so that the individual members are forgotten and the choir acts as a whole. If you have one girl on the end expressing giddy happiness and a guy who looks super intense, they may draw focus from the soloist simply because they are doing different things than the rest of the choir.

"requires deliberate thought in order to portray the emotions in the music" Only if you don't actually like singing or music. If you like singing and music, you naturally go with the emotions expressed in a song and you portray them the way you naturally would which may or may not require major facial expression changes. I say may or may not because again, some people just naturally don't have a lot of facial expressions. They look the same whether they are happy or pissed. If you have to put thought in to portraying those emotions you are most likely going to be wrong because you aren't expressing the natural emotion evoked by the song and its music. You are expressing the emotion you deliberately picked to express.

I’m am a professional singer with a masters degree in choral education. My entire life in choirs I have learned over and over again that facial expression is essential in singing. Look up any professional singer or professional vocal ensemble. As far as “natural facial expression” goes, not every song you sing in a choir is going to move you to automatically make the facial expressions the music is portraying. Your job as a performer is to make the audience feel what you are trying to say (beyond simply the lyrics) with both the sound of the music and how you look while performing it. THIS is what requires deliberate effort. Additionally, it is absolutely true that smiling throughout every song would be absurd, especially if that song were a lament or a tragic story, etc. However, that does not release you from the responsibility of understanding the emotional message in the music and attempting to convey it appropriately. The way your face looks subtly (sometimes not so subtly) affects the sound of your voice because it is changing the shape of the resonant chambers in your face that are part of the formant. That being said, if the OP is in high school and would have gotten an A except for the lack of facial expression, that is a bizarrely large drop for something that I have seen highly talented university choirs struggle with. I suspect there is more to this story.

Furthermore, the singers in the choir would NOT be making “all different facial expressions”; their job is, as a group, to have fairly uniform facial expressions contributing to one emotional message. Part of that is on the director for making sure that the emotion is clear to the singers so that they can be accurate in their performance. Different directors and performers have different ideas about the emotional message in a piece of music, which is part of the beauty of live performance. Artistic expression can be interpreted in many different ways and facial expression is an important way to convey that interpretation.

Yeah, she never mentioned it. Now she does because I’ve talked to her about it. At the time of the grade, it was still the beginning of the year and I was still trying to learn the lyrics, so I wasn’t thinking about my face. But I have classmates that talk/play while we’re supposed to be singing, and yet they have A’s.

In that case I suggest you ask for a detailed breakdown of the grading criteria and/or take it up with administration.

Most of my classmates mumble the words, don’t sing at all, or are laughing/playing during the songs. We’re in high school, so I don’t expect anything less of them. I just wish I would’ve gotten a warning so I wouldn’t have a grade lower than my peers who don’t pay attention at all

Chanting in a feminist protest isnt a professional singing career.

What does being a feminist have to do with it? I have a degree, fam.

And you’re single?! Shocker.

That's true. Ozzy Osborne only has one look. I call it his "Sharon, where the fuck am I?" face.