Having been in that position, I can say that what really helped to turn things around was to write a note with an apologetic tone, indicating that you understand that you may have come on a little strong, but that your intention was to assist the school in getting to know your child and just how capable she is. Let them know that while your approach may have been the best, your focus was then and is now, your child's education. Let the principal know that you are wanting to have a positive relationship with the school moving forward....then be sure to "catch the teacher (or other professionals at school) doing good"! Once in a while send a quick positive note...or a little candy or something for the teacher. I got to know what my dd's teacher likes and so when I am out, I might pick something up. Not all the time...maybe once a month or so. It seems like the appreciation gets me at least a listening ear.
