"miles under the school"

I grew up in a metric world, but we still use Imperial exaggerations.

It's miles and miles from here to home!
I drank gallons of water this afternoon.
That guy must be ten feet tall!
I just dragged out tons of garbage.
A one-ton truck.

etc. So (since we were a British colony and must have metrified around the same time, it was mid-1960's here), it is possible that Harry Potter did say "miles".