Can't get an older school iMac to boot from USB or even pull up start manager

Question

I've acquired four older intel based iMacs from a school, i've been collecting older computers and installing Edubuntu on them for lower income families. But I can't seem to figure out this iMac. I think because it was from a school they have a lot of the options locked down. I've tried holding down the option key to get it to show the startup manager but nothing is working. Apologize in advance but I'm not very knowledgeable when it comes to the mac OS.

Any ideas on how to get this machine booting from USB?

Answer 1

Older Macs used to use the C key to boot from the CD/Superdrive, which shoudl also give you access to the USB if the option key is not working.

However I found booting older Macbooks to give issues when using the original bootloader. In which case you could also use rEFInd boot manager, this can be installed into OSX and then allows you to choose what bootable device/partition to use including USBs. http://www.rodsbooks.com/refind/