No, due is not a preposition.
It may seem like one when you use it to talk about deadlines like you did above. However, this is not the correct categorization. When you say that your assignment is due at 5pm, for example, due is functioning as the adjective.
The noun and subject of the sentence is the assignment. The verb is a linking verb, is. When you say that it is due, the part of the sentence after that (at 5pm) is actually the prepositional phrase. The preposition there is at, which is pretty common. (Remember, prepositions tell about either the position or time order of two things.)
Due is either an adjective, verb, or noun. As an adjective and a verb, it means that its deadline is approaching. As a noun, it either means something that is owed to someone, or an obligatory fee that someone owes (dues for membership to a club, for example). It is never used as a preposition, but it is often paired with one, especially at.
