Hi, Jennifer! Welcome to our board! I'm delighted to meet you, and sorry you need you, Know what I mean??
I am not sure there is a "best" answer to this. Has anyone given your child a formal diagnosis, even if it is just ODD? If difficult child has been diagnosis'd by a doctor, then I would share this with the teacher and send a certified letter asking for your child to be thoroughly evaluated. If she has NOT been formally diagnosis'd I would probably first look at how she behaved at preschool or daycare - if she attended, or even look at how she handled Sunday School or any other type of gathering with children of a similar age.
Marg is totally correct when she says that most of us here do not believe that ODD is a diagnosis all by itself. ODD gives zero information on what is going on with a person, except that they are badly behaved. Most diagnosis's give at least some idea of what therapies are useful and how to handle them. Bipolar, for example, lets you know that medications to stabilize moods are crucial, as are certain types of psychological therapy. ADHD lets you know that certain medication and behavioral and "talk" therapies should be tried. ODD tells you that your child isn't behaving well and that the docs don't have a clue why (or don't really want to do the work needed to figure out why, from my experiences).
Given the chaos of the first week of school, you might consider waiting until the second week of school, or even the third if all is going well, to tell the teacher. Some children with problems at home show none of them at school, or not enough of them to create real problems. in my opinion this is partly because they around new people and they don't want to or are afraid to act out and/or because the structure of a school day is easier for them to handle than the flexibility of the home. Telling the teacher about your child before they get to know each other could inadvertently put your child in the "problem" category even if it turns out that she is beautifully behaved at school. There are many studies that show if a certain pattern of behaviors is expected then most behaviors that are shown are interpreted to support the expectations. (One study involved a group of people with NO mental illness who checked into a psychiatric hospital, part stating they had a certain diagnosis and part stating no diagnosis. No matter what they did, if they stated they had a certain diagnosis the staff used the behaviors to comfirm that diagnosis. The patients who stated no diagnosis were given one almost immediately and everything they did was used to confirm that.)
It would be a good idea to have your daughter evaluated by a neuropsychologist to see what problems are likely causing her ODD behaviors.