The teacher will want to make sure that the students defined the word inconceivable correctly. If the student did not, the teacher should make sure that the student wrote a story that reflected what they believed the word meant.
If the teacher notices some students had trouble determining the meaning of a word, a small group of students could be formed.
Here is some information about the word taken from Etymonline:
Meaning "take into the mind, form a correct notion of" is from mid-14c., that of "form as a general notion in the mind" is from late 14c., figurative senses also found in the Old French and Latin words. Related: Conceived; conceiving.