"Hurricane" is an early (30/40s) film that was remade recently and not nearly as well.

"The Day The Earth Caught Fire" is an early nuclear disaster with a twist. The Russians and Americans both set off nuclear tests at the same time and throw the earth out of orbit. Great special effects and look for a pimply Nick Adams (or is it Steve MeQueen?).

Spaw is right. "On The Beach" is a classic and would probably raise a lot of interesting discussion. Mass suicide is presented as the only option in the end.

How about just showing footage of Sept 11th.? A documentary was done showing the first plane hitting a tower - it was filmed accidentally during some sort of fire department training film.