Review the item:
Jackie always sat in the front row because she liked to focus on the lecture
and take good notes, the excessive spitting of her Latin
professor,
however, drove her to the back of the room.
The comma between professor and however is correct. The excessive spitting of her Latin professor drove her to the back of the room is a main clause.
However is interrupting the flow of the clause, so this conjunctive adverb requires a comma both in front and behind.