Hereford Zone: 2-4 inches
Immediate Baltimore metro area: 1-2 inches
DC, Northern VA, Annapolis: coating
Southern MD: nothing
All models are on board for rain changing to snow for Baltimore as a low passes by to our south then strengthens. The timing of rain changing to snow should be a couple hours before dawn Thursday morning. Snow should be over by mid-day (except see below). A number of model runs have placed 4-6 inches in the downtown Baltimore area. I seriously doubt this will be the case and antecedent warm conditions will largely prevent the roads from becoming too bad (pavement temperatures are currently in the 50s) and snow will likely only accumulate on grassy areas and cars. In fact, the farthest south the actual freezing line gets in any model run has been Baltimore. The Hereford Zone and other areas of Maryland near the Mason Dixon line may receive 6 inches if the air gets cold enough fast enough. As for schools, snow should be falling at dawn with a forecast of snow through the morning, so come to your own conclusions based on that information.
A big boom scenario would put 6 inches in downtown Baltimore and 3 in the DC area, but I find that very unlikely to occur and would be dependent on the low pressure hanging on the coast for a bit before ejecting east-northeast. The bust scenario is the more likely one to occur between boom/bust and that would be an all rain event, even for Baltimore, with perhaps a coating of snow at the end.
An upper level trough will swing through behind the low pressure system later Thursday around the evening rush hour. This could result in snow squalls that could produce a half inch of snow in any given location and this applies to the whole area including DC and southern Maryland.