When I was in sixth grade, I joined the band program to learn to play the clarinet (单簧管). The beginning of the year had gone ___16___. But as most students progressed, I seemed to fall behind. One day, when my teacher told us to play in front of the other students, I was filled with fear. I knew I would ___17___. When I began to play, my rhythms (节奏) were good, but my tone was another ___18___. “Did you practice your lesson?” the teacher barked at me. I felt so ___19___ and my world came ___20___ down in an instant.

上面的内容摘自2022年高考英语天津卷（第二次）的完形填空。

为什么这里的 in sixth grade 没有定冠词？表示“读几年级”可以不用定冠词吗？