I feel that some more information would be useful. Do you feel symptomatic only on school days or when school related things are happening or being discussed (ex. when doing homework, discussing grades with parents, etc.) Are your symptoms only in the morning? Have you noticed any patterns or do you have any other symptoms, even if you think they may be unrelated?
My first guess was gastroparesis, but your GES came back normal, so that's ruled out. ( I have that and you've escaped a bullet with that one). Has any tested your gallbladder through a HIDA scan? Maybe your gallbladder needs to be removed. Has any doctor ever mentioned "Cyclic Vomiting Syndrome" to you? If there is a medical reason for this, that could be it.
Have you tried any kind of anxiety pill? Sometimes they help with nausea, even if the nausea isn't caused by anxiety.