Jane and William will represent our school at the modern arts convention, for their creations have been outstanding this semester.
(A) convention, for their creations have been outstanding this semester
There is a punctuation rule that if a subordinate clause is placed in the latter half of the sentence, then there is no comma required before the subordinate conjunction. The comma is required only when a subordinate conjunction starts a sentence.
Example
Because their creations this semester have been outstanding, Jane And Willy will represent our school at the convention
Jane and Willy will represent our school at the convention because their creations this semester have been outstanding.
Both of the above examples are correct although the second one is not marked with the comma before the word 'because'.
In the given case although a school grammar teacher might fault choice A for using a comma in the latter part, it is a fact that GMAT is not fastidious about punctuations.
as far as word order is concerned, it is a thumb rule that the modifier is kept preferably close or at least as close as possible to the modified part. 'Creations this semester have been outstanding' is substantively more informative than 'creations have been outstanding' this semester. To that extent, A is inferior.