Here's why:
* Two complete sentences are combined without proper punctuation or conjunction:
* "Jessica left farm quickly" is a complete sentence.
* "She arrived at school an hour later" is also a complete sentence.
* Missing conjunction: There's no word like "and," "but," "or," "so," etc. to connect the two sentences.
Corrected Sentence:
* Jessica left the farm quickly, and she arrived at school an hour later.
* Jessica left the farm quickly; she arrived at school an hour later.
You could also use a semicolon (;) to separate the two independent clauses, as shown in the second corrected sentence.