We were using the version of System Disk that OS X installed. I'll try the OS X Server system disk tomorrow.
By the way, we weren't trying to switch to an Classic OS 9 disk, that we can get to but holding Option at startup. (We have a beige G3.) We were trying to switch to our Mac OS X Public...
As a followup,. how the heck do we get Mac OS X Server to chnge System Disks? We installed it at school on our Mac OS X box and we haven't been able to startup into Mac OS X since! Obormot and I are going crazy! Save us!