Obviously, Natsu would be one of the main characters since the title mentions him. There might also be other Fairy Tail characters like Lucy, who could be his friend or love interest in the high school setting.
In 'natsu the fairy tail highschool bad boy fanfiction', it may center around Natsu's character transformation in a high school environment. Since he's a bad boy, he might start off as a bit of a rebel, not following the school rules. But as the story progresses, he could meet someone special, like a new friend or a love interest. This encounter might change his perspective and lead him to become a better person, while still maintaining his unique charm and the magic - filled world of Fairy Tail.
Gray Fullbuster can often be portrayed as a bad boy in fanfiction. His cool and sometimes aloof attitude, along with his stripping habit which can be seen as a bit of a rebel trait, makes him a popular choice. Another one could be Gajeel Redfox. He was originally an enemy but joined Fairy Tail. His rough past and tough exterior give him that bad boy vibe.
Perhaps Natsu is seen as a bad boy because he's from a different background than most of the students at Fairy Tail School. He might come from a poorer part of town and has a rougher attitude. He could be constantly getting into fights with the rich kids who look down on him, and through these conflicts, he learns about friendship and acceptance.
Maybe Natsu is seen as a bad boy because he challenges the traditional rules of the academy. He might believe that some of the rules are holding students back from truly exploring their magic potential. So he forms a sort of rebel group within the school, and they go on adventures outside the permitted areas to discover new types of magic. This could create a lot of conflict with the school authorities.
Seilah is also a main character in this context. She has her own set of powers and a rather complex personality in the Fairy Tail universe, which can be further explored in fanfiction.
The characters in this fanfiction would develop in various ways. Natsu, as the alpha, might start off as a bit of a hothead but gradually learn to be more responsible. His friends would also grow. For example, Happy could become more independent and helpful. The other Fairy Tail characters in high school would adapt to the new environment. Gray might become more competitive in academics while still maintaining his rivalry with Natsu.
Lucy could be a main character. She was always close to Natsu in the original Fairy Tail. So when Natsu returns as an 'ex' and meets the guild again, she would likely play a big role in how he is received and in the development of the story.
He could be a bad boy because he's always skipping classes to go on his own little adventures around the academy grounds. He might be more interested in exploring hidden parts of the school or finding new magic spells on his own rather than sitting in a classroom. This would make the teachers view him as a disobedient student.
Gray would be one of the main characters to interact with Natsu. Their friendly rivalry is well - known in Fairy Tail. Makarov, as the guild master, would also have an important role in interacting with Natsu. He might give Natsu some important tasks or just have a grandfatherly chat with him about his return.
Gray would also be important. There could be a rivalry - turned - friendship aspect where Gray, who has his own ice - making powers, and Natsu, as a dragon, have to learn to work together. Maybe they compete in a friendly way to see whose powers are more useful in different situations.