At what point does the lack of modifications for dyslexic students
fall under the American's with Disabilities act?

I realize you can not provide specific legal advise, I'm just
wondering where the line is? This is the story that prompts my
question:

"I am a dyslexic (medically documented) high school senior. I have
complained since I was a freshman that foreign language was very hard
for me and I was struggling with it because of my disability. I was
very rudely treated (as if I was trying to "pull a fast one") and both
the coordinator at my school and teacher said it has nothing to do
with it.

I have had further testing that shows a direct link. I feel that I was
treaded very badly, made to feel stupid, have failed and am retaking
the class, and lastly I think my college acceptance was effected by
the grades and seeing that I keep retaking the same class.

Do I have any recourse?"

You could get an attorney to write a letter to the school for you.
Or, you could contact the USDOJ directly:
http://www.usdoj.gov/crt/ada/enforce.htm#anchor218282

I am also a Dyslexia student and the school should be providing for
you. A great resource for us is "The International Dyslexia
Association", their web address is http://www.interdys.org/. Start
with them, good luck.