This one was just okay, but it was just too similar to too many stories I've already read and seen, including the Bodysnatchers stories mentioned in the story itself, and extending to others like The Stepford Wives and The Faculty. I love the concept, but since I've seen it so many times, it's gotta take me somewhere new.
The biggest problem is that there seemed to be no reason why this random high school biology teacher was the sole expert on the subject. Is he really the most knowledgeable biology scholar on the planet? Why not talk to someone doing cutting edge biology research? When he makes up a fake miracle-drug name, why wouldn't they talk to any other biology teacher and ask "what is ____" and the other biology would say "Uh, no, that doesn't exist." And since they're earth weeds, who's to say that earth weedkiller will kill them? Maybe it'll make them turn into plant versions of The Incredible Hulk.
And the fact that he just sat and watched idly as all of his students were taken while he just sat around and watched. That's cold, man. I'm assuming that the hosts were killed by the poison as well--if that's the case, then he should at least feel a touch of remorse for all of his students' deaths. If it's not the case, well, the story could've said so.