I don't know why anyone under 40 would even put two spaces after a period. That is something we all learned to do in typing. No one takes typing anymore. I think it's called keyboarding. And I certainly hope that your computer teacher is explaining that the software knows how much space to leave after a period depending on what font you're using.
Really, this probably bothers designers far more than it bothers me.
UPDATE 2/2/14: I entered a conversation about this on Facebook during the weekend. And exited very quickly. I did not feel like debating the fact with strangers that it is, in fact, incorrect, to put two spaces after a period (unless you are on a typewriter, which nobody is.) "Either is correct," "When did this change," "I'm a two-spacer and proud." UGHHHHHHHHHHHHH!
I think arguing about it is what would give me the ulcer, so I didn't.