Though its a theory that would carry more weight, if every child there donned the same attire.
Technically there have been three costume changes thus far. Nevertheless TV live action Near cosplayers will likely choose grey pinafore trousers over a white shirt. That flannelette dungaree look remains the most recognizable, readily identifying the cosplay as Near from Death Note (2015).
However, this could equally be a more obscure Death Note costume, with the fan turning up dressed as a random Wammy's House orphan, as glimpsed on the institution's staircase in Death Note Relight 2: L's Successors (2008).
What do you reckon? Coincidence? Wardrobe staff from the new Death Note drama taking inspiration from Takeshi Obata's animation in the older adaptation? Or an actual canon Wammy House uniform blithely ignored by most of its denizens?
Who cares?! This is fodder for fan-fiction writers everywhere and probably artists and cosplayers too. Off you go!