Perhaps not quite an anecdote but if you're short of One-Liners for your student reports you could do worse than look to these examples taken from US Military OERs (Officer Efficiency Report)
- Not the sharpest knife in the drawer
- Got into the gene pool while the lifeguard wasn't watching
- A room temperature IQ
- A prime candidate for natural deselection
- Bright as Alaska in December
- Gates are down, the lights are flashing, but the train isn't coming
- So dense, light bends around him
- If brains were taxed, he'd get a rebate
- If he were any more stupid, he'd have to be watered twice a week
- Wheel is turning, but the hamster is dead