The authors describe one high school teacher's attempt to change her mathematics teaching in ways that are consistent with the National Council of Teachers of Mathematics'"Standards" documents. One of the coauthors is the teacher who teaches in a professional development school; the other, a researcher, conducted regular observations in her general mathematics class and interviewed students and the teacher. The authors describe two classes from a 10-week probability unit and present data from student interviews that were conducted approximately 5 weeks after the unit ended. The authors used these data to argue that in spite of significant obstacles, the teacher created a classroom environment that promoted meaningful, though unpredictable and not-readily-apparent mathematical learning. Although the professional development school provided a relatively supportive environment, the authors argued that this teacher would have benefitted from more consistent support as she struggled with pedagogical and content-related issues. (33 references) (Author)