Timeframe, depends if it's traditonal or not... could go year round, or the typical fall-spring
It's not at all like a normal High School... It usually has highly specialized training in various areas of interest including, acting, music,dance,instruments etc.
Often times the student can "major'(like college) in a specific area of interest & recieve special training.
It requires the student to be very passionate & involved in therir "major" & it requires alot of dedication.
Core Classes are usually taught, but usually they're based around the "major".
Students often times are required to auditions to recieve placement in such schools.
You can search all over the Web for different schools & criteria...
A really great movie on a performing arts school is "Step Up"...the focus is primarily dance, but it might kinda give you a taste of what it's like...
Hope this helps!