Whether in the British or American English standard, I have not found a lexicographic authority categorically confirming that "dormer" is short for "dormitory," but I'll hazard a guess that it's indeed an informal usage for the longer word. We can infer this from the formal definition of a "dormer" as "any type of window protruding from a roof... (whose) purpose may be to provide light on the upper floor or to add to the architectural effect." Indeed, this seems to be the context of the usage you cited, ""The Normal Hall Dormitory is now accepting male and female student dormers for A.Y. 2017 - 2017." (italicization mine). I guess it would be prudent to let that usage be since it looks like they have already internally institutionalized it in that graduate school. A "Filipinism" or not, no need to get into trouble questioning that usage since you are already studying in that school and may even end up teaching there afterwards.
