You're probably thinking of "The Fog of Courage", a CGI revival short made a few years after the fact. Eustace's voice actor was recast for it. I dunno if it was ever issued on DVD, though someone could have just recorded it and brought it to school. Watch it here: youtu.be/APT2vwudDSI
You're probably thinking of "The Fog of Courage", a CGI revival short made a few years after the fact. Eustace's voice actor was recast for it. I dunno if it was ever issued on DVD, though someone could have just recorded it and brought it to school. Watch it here: youtu.be/APT2vwudDSI
Good theory, but that short came out in 2012 so it was probably way too late for the OP.
In one of the Scooby movie VHS tapes, the original What a Cartoon Courage pilot was included, which was The Chicken from Outer Space. Eustace does transform into a chicken in that short, so that could be what you're thinking of.
Edit: I know you said DVD but I more so guarantee it had the same exact short on the DVD version too.