Depends on where you are and what the school requires.
Most simply requires a computer check... NCIC and III (Interstate Indentification Index) are two frequently checked data base. They will show every arrest, conviction where fingerprints were taken. And the results are available right away (it may take longer for them to type your letter though.) A more thorough background, if required, will involve fingerprints, which will take a few weeks.
This actually happens from time to time. When I was a cop, we have arrested a few people who came in for "clearance letters" and end up having no-bail felony warrants out for their arrest... it surely was nice that they came to us voluntarily... 8)
unless you are wanted, then they will throw you in jail!!!
Jean Luc Picard
Academic Hospitalist/Asst. Professor of Medicine, Star Fleet Medical, Earth, United Federation of Planets
Tactical Physician, Metro ESU/SWAT
In Glock, We Trust... Everyone Else... Keep Your Hands Where I Can See Them.