A thesis statement is your short description of what your entire work will be about. It typically is placed at or near the end of your introductory paragraph. Many students find creating one somewhat daunting, but it's not nearly as hard as it seems, and they are absolutely essential to creating an effective essay.
With regard to school desegregation in Boston, there are several ways to approach a thesis, considering that Boston desegregation was significant in a number of ways. Boston was one of the very first areas to undergo court-mandated school integration, long before Brown v. Board of Education. The busing system used to enact these mandates led to widespread protests, some even resulting in serious violence.
With all this in mind, and example of an effective thesis statement might be "The early court-mandated desegregation of Boston was a bold move for civil rights that led to protest, changes in public school enrollment, and even demographic shifts."