I won't get into your assumptions since others have addressed the difference between a learning environment and a work environment. That said, you are dealing with interns (not well) who are.....interns. They are there to learn since they haven't had a job like this before. You are there to guide them, teach them and help them experience your work environment. Of course they have hugely varying abilities, of course some wont be able to complete an assignment correctly -- this is all to be expected since they are interns. You may have to first manage your own expectations.

I'm not a parent to a kid who gets accommodations, but one of my good friends did in college (and before that, but I didn't meet her until college). Most of the situations that she needed extra time for, don't exist in the real work world. She had problems with fine motor skills-so things like filling out scantron bubbles or handwriting essays were very difficult for her and she got extra time or was allowed to type essays. But she was brilliant and went on to get a PhD in math. Pretty sure she has never had to take a scantron test at her job.