Pfffffffff.
No one says imperfect anymore. I see it as the past continuous in the noun phrase because the classes were a continuous action at the time of the story. That's why we use the past continuous to set the scene at the beginning of a narrative.
( The Awful Puppy Bakery is still bored, Wed 21 Sep 2011, 10:35, 1 reply)

^^^ This

(McChinaman banned, Wed 21 Sep 2011, 10:53, closed)

I was walking down the street one day,
I saw a house on fire.
There was a man shouting and screaming from the top floor,
For he was sore afraid.

BRING BACK THE IMPERFECT! Brought to you by the Classicist Old Farts Association.
( .Yeti. El amor en los tiempos del cólera, Wed 21 Sep 2011, 11:20, closed)

Oh, don't get me wrong.
I understand we still need to use imperfect to describe the mood of some past verbal phrases, but I feel that no one uses it to describe subject+was/were+present participle anymore.
( The Awful Puppy Bakery is still bored, Wed 21 Sep 2011, 11:40, closed)