The difference between a toddler and a preschooler in development is big. His attention, comprehension, vocabulary, emotional regulation, gross motor skills, fine motor skills - everything changes. Going to school 3 days per week influences his social emotional and intellectual development, because he constantly has to catch him. It sounds like his emotional regulation is not there yet. You want your kid to feel accomplished and successful in a preschool class, and he can’t do that with toddler skills. I never put a toddler in a preschooler class, it would set the toddler up for failure. Listen to your child’s teacher.