Complex sentence: She also seems to have a running grudge against Brodie, who has tenure and has Marcia Blaine for six years prior to Mackay being appointed headmistress.
Split into simpler sentences: She also seems to have a running grudge against Brodie, who has tenure. It was revealed that Brodie had been at Marcia Blaine for six years prior to Mackay being appointed headmistress.