Answer :
My school is the
oldest in our town, and it is three stories high" is the sentence with the faulty coordination. The age of the school has nothing to do with the height of the school.
The sentence that contains a faulty coordination is "My school is the oldest in our town, and it is three stories high." since both ideas that are conected are not related to each other.
Faulty coordiantion occurs when joining two sentences in a way that make their meaning illogical.
In this case, the coordinating conjunction "and" is linking two sentences that do not follow a logical sence since the first sentence is talking about school age and the second one about its hight.