One theme could be redemption. The gangster has a chance to start anew in the high school environment and leave his criminal past behind. Another theme might be identity. He has to figure out who he is now, is he still the gangster or can he become a regular high school student.
Most probably not. Usually, such titles are works of fiction that draw inspiration from various sources but aren't based on actual events. It's more about crafting an engaging narrative for the audience.
It's difficult to determine the exact truthfulness. It might draw inspiration from real-life situations but be embellished and restructured to fit the storytelling format.
One such novel could be 'XXX' (if there is a known one). In this type of novel, there are usually a lot of interesting plot twists. The gangster's past experiences would contrast sharply with the innocent and studious environment of high school. It might explore how he tries to adapt to the rules and social norms of school while still having his tough - guy instincts.
I'm not sure which specific novel you're referring to. There could be many novels with such a plot. It might be interesting to explore how the gangster adapts to the high - school environment, whether he tries to leave his past life behind or if his old ways cause trouble in the school.