It can be difficult to understand where a child’s comprehension is if he isn’t able to participate in class. Reading out loud, answering questions and contributing to discussion might be impossible.
A good way to judge achievement levels is to have parents make a recording of their child reading out loud or doing a lesson at home that couldn’t be done in class. For things like group spelling activities that need to be done in class, maybe the child could write his word or spell it out with magnetic letters. All of these accommodations should be temporary and part of a specific plan with the eventual goal of verbalizing directly. The therapist working with your student may also be able to recommend other creative ways children can display their ability and understanding of class material.