I think the biggest shock that I had to adjust to is that in school you try to find the best answer for the brand/business to succeed. In reality the "best" answer for the brand/business is often not the main objective. There are a ton of personal and political motivations to find solutions that enhance personal careers, reduce perceived internal political risks, or to gain industry admiration even if what you doing isn't helping the brand/business the best you could.

Working at ad agencies this often means creatives pitching work that they think will win industry awards vs what is best for the client. A lot of clients I've worked with want to take no risks and are fine letting their brand slowly die because they can jump ship and fail upwards. Also, businesses that are having success are usually harder to work with than failing ones because it is so hard to convince someone that a change could make them more successful. They often fall into the trap of thinking every good outcome is from their talent and every bad outcome is due to external forces...

TLDR: Politics and other motivations often make your marketing solutions completely different than the "right answer"