Unfortunately, an IEP guarantees you only a floor of services when it comes to academics and in most cases this is considered grade level work. It doesn't require that the school educate her to her potential - only FAPE "appropriate" progress.
It seems that her anxiety is holding her back - how is this addressed in her IEP? Are there accommodations in place? Would counseling teach her strategies to help her manage anxious situations? What are they doing about the selective mutism? Surely they understand how this would affect her ability to respond aloud on the spot in stressful situations. Again, how is this addressed in the IEP.
Also - part of your IEP can include training of staff so that they can understand and support her issues. Perhaps her teacher needs a crash course in selective mutism, SDI and ADHD so she can teach her appropriately.
What kind of objective evidence do have of her skill level? When was she last evaluated? Perhaps it would help if she had full educational evaluation so that you would have concrete evidence of her academic skills and this could help enlighten her teachers. If she has not had an evaluation in the last 12 months, the district MUST test her if you make a request in writing. I would ask for a full educational and psychological evaluation and be sure to have them test her reading, writing, spelling and math.
This is a tough situation. I hope you can enlighten her teacher of her potential and how the teacher can reach and support her so that her gifts are nurtured.
