The guidelines for being admitted to a mental hospital are generally simple. If you are deemed a threat to yourself or others for a reason out of your immediate control. There's a lot more to the processes and what happens after the initial admission. I know this is kind of an old post, but if you're looking for more help try checking out:
http://en.wikipedia.org/wiki/Mental_hospital
there is also a section on this site that talks about the criminal insane and using mental institutions as prisons.
Good Luck...
and if you have any specific questions about the inside workings in this type of a facility, especially pertaining to adolscents and pre-ads, I might be able to help.