Can an unexperienced artist join an art school and come out a really good artist?

A major misconception about art school: Engineering schools turn the ignorant into engineers, medical schools turn the ignorant into doctors, art schools do NOT turn anybody who walks in into artists. Ignorant here meaning lacking knowledge like it is supposed to mean. They give existing artists the environment and resources to be able to explore art, generally.

just  join now.

Define 'really good artist'. Most art schools have a style that a lot of their students come out with a focus on. A lot of art schools teach contemporary art which has more of a focus on meaning and making a statment than it does on say anatomy or painting landscapes.

If you want to go into contemporary art then yeh you might do alright depending on the school and your motivation. If you want to only do portraits I's suggest a life drawing class instead.

Another thing to note is if you mean 'really good artist' as in rich or famous then what you actually want is skills in selling your art/brand and advertising. There's a lot of artists out there, your compeating with the whole internet now, so it's your social skills that sell.

Yes!!! However, at the joining interview, they will need to see some of your recent work, in order to assess your capabilities. 

there are no paying jobs . 

Sure. That's the job of art schools. Good art schools usually require the applicant to show his portfolio of works. You might be a bit behind those students, but you can't be that far behind surely, or they wouldn't be applying to go to art school. 

An art school?  A college? Art schools, like RISD, and universities with an art college will require you to submit an art portfolio with your application. You would have to already have some natural talent and training.  You don’t join, you must apply and be admitted. The point of going to art school would be to be a better artist.  

No. No, they can't. Hope this helped.