They went there after school, & we don't know what happened between Noelle & Berdly going ahead of Kris, Ralsei & Susie - In theory, it could'v been like Kris & Susie in the supply closet where they fell down & awoke in The Dark World, or a third party knocked them out somehow.
But it'd have so be an inconspicuous entry OR they got into the library very early (Like, when it opened or later, OR they somehow hid in the closet all night since before it closed like a lunatic.) to wait in ambush.
Some suspect this was The Knight's doing, presuming it isn't Kris.
Pardon my asking, but doesn't Ralsei fail the qualifiers to make a Dark Fountain? Like, for one, we've little to no reason to believe he can be in The Light World.
Not to mention, a scarf is very unlike the only weapon a Dark Fountain has been made with so far, a knife, according to Queen. Berdly implied his Halbird could make one, too, but he may not've been aware he wouldn't have that in The Light Word.
& Knights are associated with blades.
Ah yeah. I did yammer about that in this post of mine before acknowleding that, but yeah.
I don't deny it's a little implausible, but why do you think so?