Idiot (I mean to say Unintelligent)

Oxford Says:
idiot |ˈidēət| |ˌɪdiət| |ˌɪdɪət|
noun informal
a stupid person.

ORIGIN Middle English (denoting a person of low intelligence):

Is it "ok" to call a student an idiot? I don't mean to their face, I mean in definition of their intelligence. I was trying to find a way to describe this one child, lets call her Andi. After four years of doing the same thing, she will still just stare into space... stare... stare... stare...

Trying to figure out the "best" way to describe a trully stupid person without sound offensive Oxford also says this:

THE RIGHT WORD

If you want to impugn someone's intelligence, the options are almost limitless.
You can call the person stupid, a term that implies a sluggish, slow-witted lack of intelligence.
Asinine is a harsher word, implying asslike or foolish behavior rather than slow-wittedness (: a woman her age looked asinine in a miniskirt).
Calling someone dumb is risky, because it is not only an informal word (: you dumb bunny!), but because it also means mute and is associated with the offensive expression “deaf and dumb,” used to describe people who cannot hear or speak.
Dense implies an inability to understand even simple facts or instructions (: too dense to get the joke), while dull suggests a sluggishness of mind unrelieved by any hint of quickness, brightness, or liveliness ( | a dull stare).
Slow also implies a lack of quickness in comprehension or reaction and is often used as a euphemistic substitute for stupid (: he was a little slow intellectually).
Obtuse is a more formal word for slow-wittedness, but with a strong undercurrent of scorn (: it almost seemed as though he were being deliberately obtuse).
You can't go wrong with a word like unintelligent, which is probably the most objective term for low mental ability and the least likely to provoke an angry response (: unintelligent answers to the teacher's questions).

I think my favorite is Obtuse...