Q: how do I start up in classic mode?

No MacBook Pro can use Mac OS 9 or Classic; they all have Intel CPUs. The iBook G4 and aluminum PowerBook G4s can’t boot into Mac OS 9 either.

(133952)

If it is a MacBook (MB), Pro or otherwise, it does have an Intel processor and can not make use of OS 9 for any purpose.

If it is pre-intel, it would be a PowerBook (PB) or an iBook. We would need to know the exact model in order to define what use it can make of OS 9 and how to go about doing that.

If the serial number is available, this site can identify its model designation for you -

http://www.powerbookmedic.com/identify-mac-serial.php

accidental duplicate - ignore, please