I can see what you mean. If they are making those adjustments to their grading such that it is hurting students, then a TEA complaint is likely where you need to go. You could possibly sue in district court, but in the schools I represented, there were complaint procedures you had to file, which usually means going through the TEA. Once you go through the TEA, then you can bring a lawsuit in a local court, should you feel the need to do so.
You could bring a lawsuit on the grounds that they:
a) violated their bylaws by changing the grading policy;
b) did not change their grading policy according to the bylaws; or
c) only changed the grading policy to improve the scores of certain students.
All of these things are things the TEA must investigate, but they could also be separate causes of action. The cause of action that you would have would be breach of contract. (the reason is because the school handbook/charter is their contract with the child and parent).
One of the hardest things to do sometimes is find a lawyer that you can trust. Google searches are a good place to start, but usually the lawyers who pay the most can get bumped up to be on the first page regardless of their quality. I believe that you have probably had trouble finding a lawyer because school law is difficult, nuanced, and specialized. One thing I recommend is going through your state’s lawyer referral service (LRS). The LRS in each state typically requires the attorney to have several years of practice, be free of discipline, and have adequate staffing to assist people. Here is the link for the Texas LRS - click here.
Also, if you've hired an attorney to draw up the grievance letter, but you're unsure about her abilities, my recommendation would be to have the attorney send you the letter and advise that you will send it yourself. This will give you a chance to edit and make the necessary changes. Be sure to tell your attorney you are doing this because they will not be able to put their name on it if you make changes.
I'm sorry to hear about your situation, but if you go through the administrative grievance process, I believe that you will put you and your child in the best position. Did you have any other questions for me?