If you can boot into Classic then you can boot into OS 9 stand-alone (classic just uses a valid install of OS 9 if it can find one).
To do this, you need to open the System Preferences, and choose the "Startup Disk" pane. Then choose which OS installation to startup from. To change this setting back, from Mac OS 9, go to the Apple Menu, Control Panels, Startup Disk and make your choice there.
Alternatively, on new machines, you can hold down the Option key while starting up to get a list of bootable volumes you can choose from (affects the current boot only).