Complex sentence: This was not exceptional; every corps in the BEF had one like it but the I Anzac Corps School was more influential than most, because the Australian divisions were permanently assigned to I Anzac Corps.
Split into simpler sentences: This was not exceptional; every corps in the BEF had one like it. But the I Anzac Corps School was more influential than most, because the Australian divisions were permanently assigned to I Anzac Corps.