Bombs. You forgot bombs.
Also the fact that there was only one real big alliance for the duration of the war (Sandwing, Skywing [because Scarlet had a lot in common with burn], and Mudwings [because they were afraid of burn and didn’t want to be dragged in and forced to choose a side halfway into the war]). So of course everyone outside of the group is going to hate each other, except for rainwings because they didn’t care, they just don’t want to be involved with their queen situation a mess. This is where everyone starts thinking rainwings are lazy because they didn’t get involved in the war (and mostly because of Sun Time). Icewings and Nightwings both think they are superior to every other tribe and their feud over the Icewings losing animus magic doesn’t help them. We see racism between Sandwings and Skywings in possibility but it isn’t explained further, and not explained at all in the school. The same with seawings, who aren’t used to land or being around non-seawings so it makes sense for them to have a few questions that would seem very rude. It’s pretty much a gathering of children from all tribes who haven’t seen the other tribes on good terms for their whole lives, with the only tribe being remotely nice about it are mudwings. The rainwings are actually super open, despite all the hate they get because they have enough resources that they didn’t really need to invade anyone. It would be understandable to have rainwings ask questions that they don’t think are rude but in actuality are very rude as well since they also haven’t seen the other tribes up close for a couple years (some of them will probably hold grudges against nightwings though).