I'm so glad you asked the question. When this became an issue for my sister who has two children, she spoke to their teachers and came to an arrangement that just as children were allowed to drink water in class, they could drink a smoothie provided it was in what looked like a water bottle. A wide-necked cycle bottle that wasn't transparent solved that.
So each morning the kids made themselves smoothies with yoghurt, milk and fruit and would have that in clss when they got hungry. They also took a snack with them for break which they then ate later.
