Expungement of Criminal Records

I have recently read where a federal conviction can be expunged from your record if you complete one year of community service. Is this correct?

In 1990 I had a felony case where the judge withheld adjudication. I now need to be fingerprinted with a background check for a teaching position for Lee County Schools- will this show up on a fingerprint and backround criminal check?

I have had one case sealed in Florida in 2003 for a misdemeanor charge. It is my understanding in Florida you can only get one case sealed in a life time. I have one more case and would like to get it sealed also due to the fact that I cannot get a job. Is there anything I can do to have this removed from my record. If not, would a letter of good conduct help me obtain a job? What about adjudication?

I was taken by police to the Mental Health wing of a hospital for evaluation in the fall of 2005. I was not accused of any crime. However, I've discovered that a record exists of the incident. It is now more than 3 years since the incident and I would like to remove the records. Is this possible?

I was recently denied employment because I had a Probation Before Judgment (in 2003) on a record background check. I thought this was a Non Public Record and was it proper for them to use this as a reason to not hire me.