It takes about 17 fully-grown trees to make one tonne of paper. If a school of 1000 students uses 200 kg of paper per year, the number of trees worth of paper consumed annually is approximately:

200 kg = 0.2 tonnes. Trees per tonne = 17, so trees consumed = 17 × 0.2 = 3.4 trees per year. (Paper can be recycled 5–7 times — so recycling all this paper effectively saves around 3 trees per year per school.)