Considering there may be people in your class who haven't even read the series, wouldn't it be best to start with the first book instead of trying to persuade them to read a book, which they may or may not understand without reading its prequels first?
Main points of Breaking Dawn:
Bella and Edward get married (spoiler)
Bella and Edward have bone breaking sex (also spoiler)
Bella falls pregnant with evil spawn (again a spoiler... need I continue?)
There is really no way I can think of to explain it without giving away a large portion of the book, so no matter what you do there will be spoilers.