I really liked this story a lot, and personally I didn't have a problem with the fact that it was mostly a retelling rather than a direct account of the action. I do think that "show don't tell" has moved from good advice aimed at making authors think about what they're doing into an overused, trendy dogma.
That said, I found the passivity of the students (the human ones) in this story noteworthy. They appear to be aware something is wrong and concerned about it, but all they do is sit in class and wait to be taken over. I think this is very much in line with how students appear in class - I teach at a university level, not highschool, but I definitely know the feeling that I'm talking to a room full of blank faces. But of course, the students would probably have noticed something wrong with their friends a lot sooner, and I'm pretty sure a lot of them would have reacted in one way or another. Not necessarily productively, but they wouldn't have waited for the teacher to just let them all be captured so that he can kill them later. Of course, realism wasn't really an aim here - the spores that instantly take over the human body and transform it into a plant with a cloaking device are sort of a giveaway that this isn't really going for verisimilitude. And taken as a thematic commentary on teacher-student relationships, this was just one more level of an interesting and fun story.
(Of course, if Hollywood taught us anything, is that any sort of alien - or genetically modified superplant - invading a school will be taken down by a group of students made up from nerds, misfits, and the most beautiful girl in school that events somehow contrived to draw into the action).