I'd go at this the way I address my temp history: I worked FOR the agency, but I worked AT the contracting employer.
So, I would have one main line: Local School District, March 2007 to present.
Then beneath it I would have two more leader lines, but not with a white space/hard return between them:
Lincoln School, Attendance Secretary, July 2009 to present, follow with bullet list of duties
Washington School, Secretary, March 2007 to July 2009, follow with list of duties but not exactly pasted from previous list. Surely there is some variance, if only in the rough number of students served, or one is located in a 'disadvantaged area,' or maybe you gained a new duty, lunch count, in the second position.
DISCLAIMER: I am not the Resume Expert. Further, all communications bearing my handle are my personal views and thoughts and do not reflect my employer or any official communication of my employer.