There’s really nothing wrong, whether grammatically or semantically, in your wife’s having described three of her students as “Best in English.” What that means is that she considers them to belong to that distinguished class or category regardless of whatever differences there might be in their respective grades or scores. In her reckoning, they are sui generis (to use a Latinate phrase bandied about with such relish by the protagonists in the ongoing impeachment trial of Supreme Court Justice Renato Corona in the Philippine Senate) or in a class by themselves. So there’s really no cause for worry, clementejak. Your wife’s co-teacher wasn’t well-informed in insisting that the word “best” in “Best in English” can mean only one—not two or three or whatever—among your wife’s English students. Your wife has done a perfectly legitimate classification in much the same way as such contemporary listings as “Best Brands,” “Best Books of the Year,” and “Worst Movies of the Decade.” It's a way of looking at the whole of a class rather than making distinctions among its parts. It’s admittedly a subjective way of looking of things that have a common or shared attribute, but I must say that doing so can’t really be logically or linguistically disputed.
