Think about it this way.Would the parents/student be allowed to? If they had the money, and didn't like the school-provided interpreter (for whatever reason)? Or are there rules/laws against supplying your own interpreter and declining the school-provided one?
Suppose a student or parents don't like the school teachers. Can they hire their own teachers?
Parents can complain to the school if the interpreting services are not meeting the needs of the child as enumerated in the IEP. Also, they can report unethical interpreter behavior to the principal, school board, and/or district.
Sadly, not all parents know what constitutes good or bad interpreting.