One possible version:

As is shown in the above table, senior high school students spend less time reading as their grade rises. On average, students in Senior I spend seven hours per week reading and those in Senior Ⅱ five hours. Much less time, only two hours, is devoted to reading by Senior Ⅲ students.

Reasons are various. First, most students don’t attach enough importance to curtsied reading. As a consequence, their reading time is often taken up by cither activities. Second, with their grade becoming higher, they have m ore homework to do, heaving no spare time for reading.

In my opinion, students should orange adequate reading time to broaden t heir horizons. Meanwhile, teachers may encourage t heir students’ outside reading by offering them a list of good bocks.