Even though you only have 1 class left, if you have an F-1 visa in your passport and still hava a significant amount of time for you OPT (up to 29 months now if you have a STEM degree), then you can travel, and likely you may not need the "very extreme emergency."
Ricky Malik, Esq.
More information is needed. For how long do you plan to leave the U.S. for the emergency? Will it be one week, two weeks, a month outside the U.S.? Can you resume your studies? Will the University or school provide you with documentation that it is aware of the situation? Will your academic status be jeopardized due to the trip?
If the visa stamp expires or the CBP believes that you are abusing the terms of your student visa, then you can be deferred for secondary inspection, denied admission, and/or turned around. When a person is inspected, then it is their burden to prove that they intend to follow the terms of the visa.
I strongly recommend that you review the facts and determine all challenges and risks, if any, of such a departure with an experienced immigration attorney before you travel.
The above is general information and does not create an attorney client relationship.