According to state law (TR § 21‐803.1) and Maryland State Highway Administration ASE guidelines, a school zone does not automatically appear within all roads within a 1/2 mile radius of a school, but are defined as follows:
'“School Zone” is a segment of a highway located within a School Area that is (1) routinely used by pupils for access to or egress from school buildings or grounds, (2) established by official action, and (3) designated by appropriate signs."'[...]
and
"A school zone is officially designated as such when the required signs are installed. Designated school zone signs should only be installed where school facilities are in actual use for school activities."
Standard signage is required at the beginning and end of the zones.

An inquiry was made to the Baltimore City Transportation department and a little over a week later the following response was received:
"Thank you for your request. In response to your e-mail below:
You asked if the locations for speed cameras were all pre-existing school zones. No they were not.
You asked if the city will ensure that all of these locations have required signage before the speed cameras are installed. The answer is yes.
You asked if the State Highway Administration's approval was, or will be received for designating the locations for the speed cameras. The answer is no. The city maintains the state highways. We do not need SHA's approval."
StopBigBrotherMD.org also examined several roads designated for speed cameras in New Carrollton and could not find 'SCHOOL' or 'End School Zone' signs on any of the designated roads -- both on those designated roads which were not immediately adjacent to a school and on those which did in fact pass right by a school. It is unclear at this time whether the New Carrollton locations were supposed to be school zones but were improperly marked at the time the Street View images were recorded, or whether like Baltimore the school zones were simply added after there was the new motive of adding cameras.
We have created the following videos which document this situation for 4 locations in Baltimore and 2 locations in New Carrollton. Many other locations had similar lack of signage.
StopBigBrotherMD fully believes that safety in school zone is important. However given that these locations were not previously designated as school zones begs the question whether safety concerns was the real motivation for selecting them. Other means besides speed cameras have been proven effective at reducing speeding and/or reducing accidents, but apparently those were not tried in many of these locations. Speaking as a parent of two small children myself, IF these were legitimate locations for school zones we might ask why public officials put the safety of children at risk by not bothering to do the inexpensive bare minimum step of marking the locations as school zones and alerting drivers to the presence of a school nearby... until there was a revenue motive for doing so.
========================================================================Additional Coverage as of 11/4/2009 : ABC2News has investigated this further. Apparently cameras are being installed before the signs are going up. Referring to a new camera on Norther Parkway "Workers installed speed cameras at the intersection today, the signs formally calling it a school zone will come later. " See TV Story.