In high school, I had a German immigrant, fresh out of college as an instructor. The moment one stepped into that classroom to the moment one stepped out, nothing but German.
Hell, it didn't stop there. If you tried to talk to him at all, in or out of the classroom, he wouldn't acknowledge you unless you spoke to him in German, and he would only speak German to you.
Strangely enough, the German students were the only ones capable of holding conversation in a foreign language.
I graduated in 2008, haven't 'used' German in 7 years, but I still find myself thinking in German every once in a while.
Immersion (even for ~50min, 5 days a week) is easily the best way to learn. I don't know why anyone wastes their time with anything else.
EDIT: Hah, I just remembered how he never cared that we talked to each other during class about whatever, just that it was in German, not English.