Once exposed, you won't know whether your child has chickenpox for at least 10 days and for as long as 21 days. That's the waiting period for the first sign -- red spots -- to appear. These spots start on the chest, belly, or back. They are raised, red, and become fluid-filled within 24 hours or so. The spots are often itchy and seem to spread all over the body quickly. There can be hundreds of them. She may run a fever, not eat as much, be bothered by the itching, and just feel "under the weather."
If you think she has chickenpox, keep her home from school until all the spots are crusted over -- usually about one week. If there is no sign of the disease after 21 days, your child has escaped for now, until the next exposure.
About rushing to have her vaccinated -- I am not aware of evidence that having the vaccine right after exposure will prevent chickenpox.