Black Eye Guy,
I think some tv show did tests on a person getting out of the grave, because of the weight of the earth it would have been impossible. Plus as soon as a hole was made in the coffin the dry earth would have dropped into the hole. When you saw Buffy just after she had risen she was a bit dirty but she wasn't muddy so I'm guessing it hadn't rained on the grave in 148 days, you'd have thought the green grass would've been brown by then.
Now I'm guessing that she tried to get out of the coffin as soon as she was given her life back, so the air factor in the coffin doesn't enter into it. Although Buffy would have had an alterior motive, it would have stunk like bloody crazy in there. Escaping bodily aromas and all that.
And the scoobs must have known she'd be six foot under, you'd have thought they'd have gone to the funeral.