What does your W-4 say and what kind of tax documents your signed with the
school?
Since you've been in U.S. for more than 3 years, that tax treaty does not
apply to your case. Your tax residency status is RA, I guess it might be
your school withheld NRA additional withholding tax? Details see page 21 of
publication 15 http://www.irs.gov/pub/irs-pdf/p15.pdf
