I think this was not so uncommon, although often phrased differently -- in many places conscription was obligatory, but could be deferred while in university...

I think this was not so uncommon, although often phrased differently -- in many places conscription was obligatory, but could be deferred while in university... and after graduating you'd be useful enough not to spend 2 years digging ditches. And perhaps by the time you had a PhD would be exempt.