Perhaps the antagonist could be a teacher who has a dark secret. This teacher might be using the students for his or her own gain. Sasuke and Sakura notice some strange behavior from this teacher and start to suspect that they are up to no good. They then become the targets of this teacher's wrath as they get closer to uncovering the truth.
The plot could involve Sasuke and Sakura being outcasts in a dark high school environment. There could be a powerful bully or a group of them that they constantly have to fight against. Along the way, their relationship might develop as they support each other through these tough times. They could also be involved in uncovering some long - forgotten secrets about the school that are related to dark magic or something equally spooky.
One possible antagonist could be a popular girl in school who is jealous of Sakura's popularity and tries to undermine her at every turn. For example, she might spread false rumors about Sakura or try to steal her friends. Another could be a strict teacher who doesn't believe in Sakura's abilities and is always giving her a hard time, like constantly doubting her work in class or not allowing her to participate in certain school activities.
There could be a mysterious organization that has been hidden for a long time. They emerge when Sasuke and Sakura start their journey. This organization wants to use Sasuke's power and Sakura's medical knowledge for their own evil purposes. They set up traps and send assassins to target the duo throughout their journey.
A strict teacher could be an antagonist. Maybe this teacher has old - fashioned views and doesn't like Percy's unorthodox ways or Artemis's independent streak. The teacher could constantly give them detention or try to stop them from participating in certain school events that are important for the plot.
Gary could be an antagonist. He might be jealous of Ash's relationship with Dawn and try to cause trouble between them. Maybe he spreads rumors about Ash in the school. Another possibility is a popular student who doesn't like that Ash and Dawn are getting all the attention during a school event and tries to sabotage them.
It could be a story full of teenage angst. Sasuke might be the brooding loner, while Sakura tries to break through his walls. Maybe there are emo bands playing at the school dances and they share a moment during one of those.
Naruto is often a popular character in these fanfictions. He could be Sasuke's rival for Sakura's attention or a good friend who tries to help them get together. Ino is also a common character. She might be Sakura's friend or sometimes a rival in matters related to Sasuke. Kakashi could be their teacher who gives them advice about their relationships and schoolwork.
The school bully could be a main antagonist. He might pick on Austin for being a singer and think he's too full of himself, or he could target Ally for being a bit of a nerd. Another could be a jealous rival singer in the school who wants to take Austin's place and will do anything to undermine him. And there could also be a strict teacher who doesn't believe in their music dreams and tries to stop them from performing at school events.
In the Sasuke Sakura high school love story, Sasuke is often the cool and aloof type. Sakura has a big crush on him from the start. Sasuke, at first, may seem indifferent but gradually he starts to notice Sakura's kindness and determination. Their love story might involve things like Sakura bringing Sasuke lunch, and Sasuke protecting Sakura during school events.
A group of troublemaker students might be the antagonists. They could be constantly causing chaos in the school, and Naruto and Sakura have to step in to stop them. These troublemakers might not like the new order that Naruto and Sakura are trying to establish in the high school.