I am reading your letter and as a school psychologist I am wondering a few things-
First, is your daughter perhaps placed in classes that are too challenging? Sometimes, this happens. Does she have a history of academic problems in school? Has she received special help in the past and is not now?
Second, has there been a change in her life that is significant. This could be a change within the home (such as a divorce or marriage), maybe a with her physically (has she been ill, experiencing fatigue that is uncommon for her, etc.) or has become involved with new friends or a boyfriend?
I would recommend that you start by talking to your daughter and ask her what she thinks is occurring. You also may wish to talk with her doctor to rule out any physical problems and the school guidance counselor.
Monitor her behavior and activities to ensure that she has not gone astray and is not engaging in risky behaviors such as drinking or drug use. Many parents are surprised to find out that their child is engaged in these behaviors and the only "tip off" was a change in school performance.
I am adding two helpful websites written by specialists who work with teens.
Louise Masin Sattler, NCSP
Nationally Certified School Psychologist
There are places like Learning Rx that test children. So you really know what deficits if any you are dealing with...
Can the child read? That's so basic, but it's very common in American schools to use methods that don't work (Whole Word, etc.) Quick test goes like this: does she drop out words, add in words, substitute words OR reverse words? If so, then you have to consider starting over with phonics. If school has used term dyslexia, see article: