Sure,you can practice on your own all you want,but as Ghetto said,it's not going to prepare you for reality.School teaches the basic fundamentals and if you want to work in a "nice" restaurant,you better know them.
Chef asks you to cut those carrots in a fine brunios or tournet 50 lbs of potatoes..you better know what they mean.Chef asks you to make any/all of the mother sauces,can you? A consumme? Pate En Croute? Basic forcemeat? You'd spend a lot of $$ making these things at home over and over again to get them right
If you're serious,go to school.It would take years and many a patient chef to teach you all you need to know,and most "nice" restaurants won't even hire you as a prep cook unless you have a degree.
School isn't just about cooking;it's about safety and sanitation classes,Basic English,French,Spanish,Psychology,Business Management,food costing,recipe conversions [metric and standard] nutrition classes,knowing the history of all cusines and the regions they come from,proper knife techniques....but even school doesn't teach you everything.For every one thing you learn in a safe,sterile demo kitchen,there are 50 more things you learn on the job and each chef has their own way of doing things.
But just as true is the fact that a degree will open a door,and sometimes only a crack.It doesn't gaurantee that you'll land some high-paying job somewhere in a respected kitchen and you will not have "Chef" status;that takes years.And keep in mind that the recent statistics are less than 10% of grads will still be in the business 5 years after graduation...and they still owe the money for the student loans.