I dont have this problem, as I am doing this myself at home. I weigh tomorrow morning.
When I did go to class, I was being weighed in the evening, so I would have a salad for lunch with a Muller yoghurt for dessert. My last drink of water would be around 3pm (I was being weighed at 6.30) and that was enough for me to see the results. I never went on a Green Day the day before or the day of weigh in either.