It looks like this isn’t just one my pet peeves. Farhad Manjoo of Slate Magazine offers irrefutable proof why you should NEVER EVER use two spaces after a period.
If you don’t have time to read the full article the only reason two spaces became popular is because of mechanical typewriters which weren’t able to deal with uniform spacing. Since the age of the electronic typewriter in the early 70′s there has been no need to add two spaces.
The biggest reason why there are teachers still teach two spaces–because that is what they learned.