This study explores the use of Snapchat among teenagers in France, examining how its unique features shape social dynamics and trust among young users. The research was conducted between 2016 and 2018 in four middle schools located in Parisian suburbs, and included 150 interviews with pupils aged 10 to 17, a questionnaire with 236 responses, and nethnography of 100 Snapchat profiles. Additionally, education professionals and the French Snapchat CEO were interviewed. The findings reveal that Snapchat is used by teenagers to test their peers' trust boundaries, resulting in significant changes to friendship dynamics. This is illustrated through the use of snap streaks, where teenagers ask friends for their passwords to maintain the streaks. Refusal to share passwords can lead to suspicion and mistrust, creating an environment of secrecy and betrayal. Despite the ephemeral nature of snaps, teenagers find ways to save them, leading to the perpetuation of rumors and misrepresentation of facts. The study highlights how abusive trust on Snapchat can result in a loss of reputation and negative consequences for users. Through an analysis of cases of betrayal and reputation shifts, this paper illustrates how private sharing on Snapchat creates an ambivalent trust relationship within teenage friendships.