You can’t just substitute all the flour for cocoa powder. You could maybe do 3/4 flour and 1/4 cocoa but it would really depend on the recipe that you are using.
Sorry that answered this so late!
Really it all comes down to being willing to try new things, new recipes because this is the place to do it. You won’t get fired for screwing something up in culinary school.
That and ask tons of questions. The chefs have a tons of experience (and connections) so really make sure you utilize that to your advantage.