One of the rules of CAS is you can't 'double-dip' and use anything that is part of your academic programme also for CAS.
However, you can be inspired by something you do in class that makes you want to go further and make a change in the world or to apply that knowledge for real.
If you are passionate about an academic subject and want to study it further, say at university, completing a CAS project or experience connected to it will prove this.