老師話我係descant,但我係school choir係soprano,但係我又想做mezzo soprano wo!

由於唔知你聲底係點，憑你的形容，只是略作推測，未必那麼真確。
唔單只係音樂，好多學問同技能都係，就係話先天後天各佔一半，
問題如屬先天，無法改 變，其實都冇咩討論價值啦，因為各人天賦本錢不由得自己決定。
但後天努力卻可憑自己能力去改變，通常都係靠努力學習。

其實如果考唱歌，soprano係難考過mezzo，所以考完mezzo先好去考soprano就穩陣d，因為soprano D曲目深好多。

照你講，你對自己唱mezzo冇咩信心，如果係真，你個老師卻話你可以越級挑戰descant，咁o既講法好似怪怪地。

照我估，可能你係雞仔聲，加上唱歌容易受其他分部影響，唔適宜唱一D"兩面受敵"o既聲部，姐係mezzo(會受高音部及女低音上下夾擊，好似三文治咁)。
所以老師叫你唱descant，一來descant需要d比較清脆的聲，唔駛咁厚，二來又容易攞音(最高果部咪係囉)。

如果想練厚d把聲，其實你有參加合唱團的開聲訓練就已經可以改善聲質，不過一兩年貨仔係唔會咁快有改善架，好多人都係唱左超過五年或以上，唱歌技巧先會慢慢純熟。

Most probably it is born like that. I am tenor, but I also want to sing Baritone, since I can't sing too high (I can go up to only B now).

if you want to 練厚聲, you MUST sing with full support, and keep singing low register over and over, and you may try listen to Basses (not alto), and try to sing like them. (Originally I could sing up to only G, but I keep trying to sing like Domingo and Pavarotti and then I find myself easier to sing high notes). After certain practises, you will find your voice stronger GRADUALLY.

But you are descant, you should be able to sing very high notes, then why do you want to change part? If I could sing even higher, I would love that.

Most important of all is that you have to avoid 走音 when you are singing low notes (which means practise), then you may ask your teacher if you could be mezzo soprano