sub-grade in saline soil region

例句：

1. 1.The sub-grade students were given extra help in mathematics.
   次级学生在数学上得到了额外的帮助。
2. 2. The sub-grade teacher was very strict with the students.
   次级老师对学生非常严格。
3. 3. The sub-grade level of the textbook is too difficult for most students.
   这本教科书的次级水平对于大多数学生来说太难了。