(1) Number of books donated in Grade 9: 1000×30%×4= 1200 (Ben).

The number of books donated in the eighth grade is: 1000×35%×6=2 100 (volume).

The number of books donated in the seventh grade is: 1000×35%×2=700 (volumes).

∴ The total number of books donated is:1200+2100+700 = 4,000 volumes (volumes).

Therefore, the total number of books donated by students in this school is 4000.

(2)4000÷ 1000=4 (Ben)

So the school donated an average of four books per person.