When my son was in Montessori preschool he was exactly the same way...what was happening was that he was getting overwhelmed and distracted by everything that was going on around him (he definitely has OE issues) to the point that he ate like a mouse. He was always so skinny because he is not a breakfast eater and he would pick at his snacks I sent in (he helped pick out his snacks and lunch and it was always regular stuff he ate at home) and then he would barely eat any lunch. When we eat at home we like to sit and talk and meals are very relaxed and laid back but at school everything was rush rush rush to get through lunch. I started to go into school and would sit and eat with him and I could easily see why he was so distracted because lunch always seemed to be a zoo. Now that we homeschool it is not an issue but we went through the same thing
