Why are terrorists targeting schools and universities?

Over the last 13 months there has been a series of extremely bloody attacks on schools, colleges and universities. On Wednesday Taliban suicide bombers mounted an assault in north-western Pakistan; just over a year ago the same movement attacked a school only 30 miles away,killing 150 people, most of them children. In April last year 147 were killed in a massacre at a university in northern Kenya by the Somalia-based al-Shabaab.
Then there are hundreds, possibly thousands, of other incidents of violence directed at educational institutions, teachers or students across the world which have not been reported.