First of all, are you 100% sure the computer at school will play DivX? DivX is not installed by default on Windows machines so you might want to double-check before Wednesday.

As for converting, just use VirtualDubMod, which should have come with your Gordian Knot download.

Open up your video in VDubMod, and go to File -> Save as...

At the bottom, make sure "Full Processing Mode" is selected from the drop-down menu, and click on "Change"

Select the codec you want to convtert to and click on "Configure"

For a 30 second clip, you probably don't need to do a multi-pass encode, so just choose a 1-pass setting.

Configure the settings as needed or leave them at the defaults if you're not sure. Then save to a new file.

Hope that helps.