Unfortunately there really are no rules or requirements that dictate proportional representation for any taxing bodies, unless there is something specifically written in the by-laws of the individual school or taxing district that makes this a requirement.
If you feel that representation among the school board members is biased due to membership coming characteristically from wealthy wards, then your best recourse at this point would be to start a petition drive among the individuals affected by this local taxation, and present the petition to the President of the school board, along with a copy to your local alderman and to your local state representative.
Since the school board members that you refer to are "elected" members, it may make it more difficult to actually have any of the members removed from their position, since they were not appointed by any one person or any one group. However, hopefully by filing a petition, before the next election term comes around, they may change the by-laws in such a way to ensure that all of the wards have a chance at equal representation.
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