I support Fukuin having male kids on the logic some have stated that it'd have been ridiculous for nobody to question Kanon if it was a strictly all-girl place. I think ... we may only be having this discussion because of happenstance on Ryukishi's part.
Other off-duty servants have been mentioned since the beginning, but we had no idea what gender they were. The only female servants on duty during the conference were Shannon and Kumasawa, who's only part-time and had been there forever. Anyways, so I think he got to Requiem and decided to show us other servants, which ended up being the Meido's of Purgatory, which I saw partly as fanservice, partly as continuing to show the origins of Yasu/Beatrice's magical compendium (i.e, she based her servants on the other snobby girls who worked with her), and there was no room to show a male Fukuin servant, similarly, into the scenario. Whose heads would he have copy+pasta'd for them?
Um ... on Jessica's surprise at a boy servant, it's possible that, despite being a co-ed orphanage, in general, girls would work harder to achieve the grades / behavior that put them into the running for the position. This could go farther and say that Jessica had seen male Fukuin servants before, but Kanon was the first she'd seen since she started, y'know, noticing boys.