a) The storm came up suddenly with a loud boom of thunder.
b) Most raced to the school, but others just walked.
c) At first, the sky and the streetlights were dark.
d) The wind vibrated the trees and whistled past the buildings.
e) We huddled together and held our breath under the leaky roof.
Explanation:
a) Combine the subject of both sentences to make one subject (the storm) and combine the predicates of both into one simple sentence.
b) Make it a compound sentence by adding a comma and the conjunction "but", and by taking out the second time "to school" is mentioned.
c) Combine the subjects of the two sentences (the sky and the streetlights) to make it a compound subject.
d) Combine the predicates (including the verbs) to make it a compound predicate.
e) Again, a compound predicate is formed by combining the verbs and predicates of the sentences.