CHICAGO — Jones College Prep students gathered for a sit-in Monday following controversy around a student who attended the school in a German military costume for Halloween and made gestures seen as antisemitic. The school’s lack of response to the incident prompted the protest, where students called for change in the school culture, saying administration has failed to properly address long-standing discrimination issues. Students packed the school’s lobby, with many others watching from the second floor. A few students held signs toward the windows facing the street, some of which read, “You …