Could be so many things - stress, anxiety, someone in the class, something that happens just before class, the after shave or perfume worn by the teacher, the detergent used by the child who sits next to her, molds in the ventilation system in that room, etc. A lot of postnasal drip causes stomach upset - could be the time of day, what else is in her stomach (or not in her stomach) at that time of day, or more. I have a friend who did amazing work shutting down a school where so many kids and staff got sick - she was an amazing advocate. There is a supplement you can give her that may help but if that doesn't work, you should insist on some environmental testing in the classroom - what cleaners are used, what's up above the ceiling, what's in the vents and filters, what's in the pad under the carpet (if there's carpet in the room or the room she's in just beforehand), and more.