I think the misunderstanding comes from the phrase, and not the intention. To me, they might have been referring to him not being able to sit still long enough or have a long enough attention span to ever get married or hold a job or whatever. You read it as an attack on his reproductive system. YMMV.
I actually read it as the teacher was proclaiming the student was too homely, too ugly, or (I guess with the number of molesting teachers these days) wasn’t equipped well enough. Maybe they were making fun of how the girls in his class avoided him like the plague.
Honestly, I never would have (and still don’t) see “not being able to sit still long enough or have a long enough attention span to ever get married” as a viable interpretation for the award.
But regardless of their intent, these (college-educated, I hope) teachers should have forseen how “Most Likely Not To Have Children” could have been interpreted.
(Note: I am not condoning the victimhood claims of the kid’s mother and stepfather, but the “teachers” should in no way be absolved of this stupid incident.)