I was out sick for two days and overwhelmed with work when I got back.  I am not complaining about the work load; I was able to catch up on things eventually and before end of the day, I was able to input and log the documents.  The last thing I have to do is distribute the papers.  Although with a huge hesitation, I sought the help of this co-worker so the papers would get to the recipients on time since it is a Friday.  As expected she had numerous stupid questions.  After spending time answering - as much as I can, her stupid questions, it ended up with her not helping me.  She passed on the responsibility to some students helping in the office.  Before, this assignment was a our joint responsibility.  Since it was so difficult getting things done on a timely manner from her, I asked to take the responsibility entirely.  I developed a system and things ran smoothly and I was able to deliver efficiently.  But then I got sick and the paperwork piled up.  FYI - Overtime is not an option. 
Our bosses do not approve of her work attitude either to the extent that our Principal openly insults and disrespect her.  I don't blame the bosses because, this co-worker has no sense of discretion, no work ethics and unprofessional.  She is always late and would blame traffic lights, not getting much sleep, etc.  The most important part of her job is payday.  On several occasions, she was caught sharing confidential matters to parents prior to their meetings with school authorities.  
Outside work, this co-worker is very caring, sweet and I love her to death.  What do I do to deal with this situation?  I can't discuss the issue with the administrators; I am afraid she could get in trouble because of prior incidents.  I've discussed the problem with common friends and tho they have the same impression about this co-worker's work ethics, I was not able to get any help.  I feel really frustrated.