My pal Emily Lakdawalla from The Planetary Society has started making a series of short videos about astronomy and space science. She's made one about NASA images, seasons on Mars, and now this one on asteroid impacts:
And she's wearing an SMBC shirt!
These are pretty good: short, informative, and I'd bet useful in the classroom as well. Emily writes The Planetary Society blog, too, so you should make sure that's in your RSS feed. And subscribe to their YouTube channel, too!