What grades are the children now? How much time has passed since the beginning of the novel?

Jem was in the seventh grade and went to high school, beyond the grammar-school building; I was now in the third grade, and our routines were so different I only walked to school with Jem in the mornings and saw him at mealtimes. When we started, Scout was in the first grade, and jem was in the fifth. Thus, two years have passed.