In Reception class when they got their free milk there was a special "milk" table (plus a water table). The milk cups had to stay on the milk table, the allergic child didn't go near the Milk table, and all the kids who had milk washed their hands and faces before being allowed to go back to activities.
Higher up school, DD chose not to have the food her friend was allergic too in her lunch box so she could sit with her friend at lunch.