In which year the percentage increase in the number of total students in schools A and B taken together is the highest in comparison to its previous year?

Total students in School A and B together in 2008 = 640+550 = 1190

Total students in School A and B together in 2009 = 800+820 = 1620

Total students in School A and B together in 2010 = 500+600 = 1100

Total students in School A and B together in 2011 = 700+750 = 1450

Total students in School A and B together in 2012 = 900+500 = 1400

Total students in School A and B together in 2013 = 750+480 = 1230

Percentage increase in 2009 from 2008 = $$\frac{1620-1190}{1190}\times100=\frac{430}{1190}\times100=36.13\%$$

Percentage increase in 2010 from 2011 = $$\frac{1450-1100}{1100}\times100=\frac{350}{1100}\times100=31.81\%$$

There was decrease in number of students in 2010 from 2009 and 2012 from 2011.

$$\therefore\ $$Percentage increase in 2009 from 2008 is highest

Hence, the correct answer is Option B