When my ds was in second grade (last year), he stared going to a gifted magnet school. The bus ride was about 25 minutes each way. The year started out okay, but then we had an incident of bullying in the classroom and on the bus that was resloved when I talked to the teacher. He was being teased by an older child on the bus that the counselor straightened out. Last year, a girl on the bus told him she was going to set him on fire when the time was right, and this year the same girl told him nobody likes him. I don't know this girl well, but I do know the mom left when she was small. Maybe that's part of her problem.
My ds is a good victim because he reacts to the other kids. He probably sticks out in a number of ways (small, geeky, immature), but I think the biggest attraction is how he reacts and takes things literally.
I work in an elementary school (not his), and I know that kids can be mean, especially girls. It starts in 1st grade or earlier (you can't come to my birthday party) and increases from there. I think the girls gain power by intimidating others. It happens with boys too, but not to the same extent. Where do they get it? Maybe from home but also from other kids.
For my ds, we talk about what happens, and he's starting to see in retrospect how he could handle situations better. I think it also helps that ds is finally making friends. He's involved with a small group at achool, and those kids have bonded.
I don't know if I have any advice for you. I offered to allow my ds to go back to his home school, and he's not interested.