'Drill' in current usage almost always refers to drill music — a hard-edged subgenre of hip-hop that emerged in Chicago around 2012 and developed UK and New York scenes (UK drill, Brooklyn drill). The lyrics frequently reference street violence and rival gang conflict, and the genre has been controversial, including in UK courts. If your teen listens to drill, that is mainstream; if they are involved in making or sharing content that names specific rivals, that warrants a serious conversation. The music itself is widely consumed and not inherently dangerous to listen to.