There are two answers to your question.
For starters, the timeline from a written request for evaluation is 15 days in California, and I believe that is the timeline in federal law.
However, it should really be a moot point.
Although you always want to have an accurate assessment and eligibility designation, disability should not be driving decisions.
Once a child has an IEP, regardless of the disability, the IEP Team is charged with creating goals and supports to address any impediment to accessing a meaningful and measurable educational benefit.
If they are claiming that your specific concerns are not preventing educational benefit, it would be based on two potential explanations.
1. Student is getting a meaningful and measurable educational benefit, so they believe that the IEP is doing its job and is defensible. In that case, they should be able to provide you with ALL of the measures that lead them to that conclusion as well as the evidence that would contradict this… (grades alone should be inadequate, and education is more than just academics… future employability, ability to work in groups, ability to live independently are also educational outcomes).
2. Student is not getting an educational benefit, but that they have evidence that the reason you give is not the reason why. In this case, they should be providing you with the evidence as to why their IEP is reasonably calculated based on the information available at the time to provide a meaningful and measurable educational benefit in the Least Restrictive Environment. In this case, their actions are not judged on whether or not you are right or wrong, but whether or not they can justify what they are doing.