Quite a few people prefer to paint a building even if the plastic color is correct. If you choose not to paint it, at least try to weather it a little with a diluted wash. This will cut down on the plastic look.
I looked at the kit on the Walthers web-site. The red in the picture looks like it’d be a good match. It’s looks similar to the red used on barns back in the day. It would make sense to have a similar red on a school house. Just my thoughts.
Well, to answer your second question, red w/ white trim is fine, probably should dullcoat the building to remove any plastic sheen, and maybe add a bit of (airbrushed) weathering around the foundation and by the door/stairs. This admittedly unscientific sample of Google images seems to indicate red w/ white trim was second only to white w/dark trim for one-room school houses. Google image search.
But my post was to indicate to you that, in fact, there were very few one-room school houses in operation by the 1950s in the Eastern US, and that your school building would be serving as something else by then (maybe a residence, or a church, or a store house) or would be unused (in which case it probably would be in somewhat ratty shape), or maybe would have been converted into a museum by then (from wiki: “In Iowa, over 125 small one-room school houses have been turned into local museums.”)
That was an eye-opener. I was first concerned about the colour, now I have to figure out what I’ll transform this house into - a museum is a nice idea!