One story could be about a student who was held back because of excessive absences. Maybe they had a family issue that made them miss a lot of school. They felt really left out when they had to repeat the grade and saw their friends move on without them.
I don't have enough context to determine exactly who 'garp' is. It could be a newly created character by the fanfic writer, or it might be a character from a relatively unknown story that is being incorporated into the 'ace fanfic'.
It's hard to say. Sometimes these events get postponed or cancelled at the last minute. I suggest you contact the organizers or look for updates on social media.
It's hard to say exactly where Comic Con is being held this year without the latest updates. But often, it takes place in well-known convention centers in big cities. Check with relevant event organizers or entertainment news sources for the definite location.
Poor academic performance is a very common reason. If a student fails a lot of tests and assignments, they might be held back. Another reason could be behavior problems. If a student is constantly disrupting the class, they may not be able to progress to the next grade. Absences also play a role. If a student misses too much school, they will miss out on important learning and may be held back.
It could be the village elders. They often have a lot of power and influence in the Naruto universe. If they believe Naruto is a potential threat or they have some ulterior motives, they could be the ones holding him back, thus acting as antagonists.