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... he had a tough day today regardless because the sub didn't give him any of the accommodations he is used to having and he had trouble paying attention... He usually gets to skip work he knows how to do, and, while it's important to be able to handle doing work one doesn't feel like doing from time to time, it is hard for a kindergartener to understand why they suddenly have to do work they usually skip.
Some schools have a plan for seamless transition to substitute teachers: special notes (such as accommodations) are included in a ring binder to quickly raise awareness of compliance for the substitute teacher. Paras or teacher aides may also be available to assist if the sub is unfamiliar; This ensures continuity for the students in the teacher's absence.

Was this the only day for the sub? Will the classroom teacher be back tomorrow?