This isn't a slam against your professor but, when it comes to business, nothing beats real life experience. There is a lot you can learn by diving into an entrepreneurship that you could never get from a classroom.
Consider these lessons:
You Are Your Schedule
Show of hands: who has ditched a college class? Everybody, right? It's hard to imagine getting through four years of business school without sleeping in at least once a semester. Out in the real world you won't have that luxury. You'll learn very fast that your work schedule is what will dictate your potential for success. This might mean taking a meeting on a Saturday morning or working into the wee hours to get a project done. You don't have a lot of options because running a business doesn't mean you can get a "do over" test or hand in a paper late.
Say Goodbye to Down Time
There's a reason why Spring Break is so vital to college students: It's an official break. A week without worrying about tests or papers or classes. In your business you can take a vacation, but chances are during that first year of the startup you'll be putting in some serious work hours and vacations will be on hold. This doesn't mean you'll be working around the clock (not all the time at least) but you'll come to value that time you can take off a lot more.
Keep Your Eyes on the Prize
A typical college semester could be filled with a variety of classes. Yes, they're all going for the same degree but you'll be multitasking up a storm as you juggle multiple classes, homework, projects and tests. For your business you'll benefit from those multitasking skills you developed but you'll also learn the value of laser-like focus. Always keep the big goal front and center and learn to delegate.
Go Beyond the Books
As a business major you'll be required to take some basic accounting courses. The key word here is "basic." In your business, you'll be getting a crash course in whole new level of accounting with regard to taxes, payroll, insurance and shelters not to mention maximizing profits and adjusting for losses. That's a lot of numbers to keep track of and no, you won't be graded on a curve.
Business Relationships Trump Frat Buddies
College is a time for fun. It's the first change students have to live on their own and make decisions. Usually that means a lot of parties. Once you've graduated, you'll need to start building business relationships. Some of your fellow classmates might be a benefit in terms of networking connections but it's vital to strike out on your own and create forge new friendships with people who can help your company.
If you're in college now, embrace the good times because it's an upwards climb from here on out!