The best art schools are selective, because a lot of people want to attend- and they want to attend to learn the advanced stuff, not the basics. The art school has the right to pick who they feel is most likely to succeed at their course. It is a huge waste of the school's time and resources, and more importanlty, on the paid-for tuition time of the other students, to use time out of a course intended for people with a high skill level, on teaching the basics, when the people who the course is intended for already know that stuff.
That is why they need a portfolio- to sort out who is ready for what level of training- and if they only offer advanced training, this means that the majority don't make the cut.
You can pick up even advanced skills by mucking about, and referring to the internet. If you need structure, set yourself a structure and stick to it- if you can't stick to it, you may not be dedicated enough to be suitable for a course anyway.
I doesn't really work that way, dedication and discipline are to different things. Many people are dedicated enough to learn, but not many are capable of teaching themselves, end even if you are, you will never learn as much as you can through instruction.
Instruction can be found by other means though- with something as popular as art, there is a huge amount of material available, largely for free via library or online, that is there to teach people. There are online boards, where you can get good feedback on your progress, which I'd say is 90% of the benefit of courses. The other 10% would be the materials and tools available, but you can buy a lot of supplies with the money you'd spend on attending an art school.
I've had very good luck with books, the internet, and simply watching skilled people work... my experience of art classes was that it's often a waste of time, with most of the time and the teacher's attention spent dawdling about at the pace of the worst students, so they'd 'catch up' so most advanced topics are barely glossed over.
I've had a much different experience. The downside of the stuff your talking about is that, with something as popular as art, is that there are so many different material meant to aid you that it can be somewhat jaring for a person to find what will work for them, and that can lead to someone giving up. With a class you are able to get aid from instructors who have experience with your strength and weaknesses.
*nods* I guess teachers vary, and people have very different learning styles. I tend to enjoy the research and find it to be a quick way to assimilate new information, and am usually aware of my weak points, so I find the negatives outweigh the positives for me, but I can understand if that's not the case for you.