I assume that Kirschstein was still a pupil or at least finished school when the war break out. He was just 18 years old in August 1914. According to Roneingh: "Zwei Generationen Luftwaffe" his father was Regierungspräsident von Posen and Kirschstein entered as a Kriegsfreiwilliger 3. Pionier-Bataillon "von Rauch" in Spandau. Did you check Hanns Möllers opus magnum about the PLM-holders. Perhaps there are are more infos to find ???
Wenn man den Beruf seines Vaters in Betracht zieht, scheint es wahrscheinlich, daß er sich vor dem Heereseintritt noch immatrikulierte. Wilde Spekulation...
Gruß, Immo