The first time she brought her 4-year-old to Mendell Elementary School, Ellen Shattuck Pierce braced herself for disappointment. She knew little about the small Roxbury school in Egleston Square; she had never heard other parents talk about it. Whatever the opposite of buzz is, this school had it. “I thought it was going to look desperate, because I was desperate at the time,’’ Pierce said of her first visit. “Instead I found a lovely, cheerful, well-lit place, where kids were happy, learning and engaged.’’