The school have asked me to prepare UHT milk for my girl. How long can I keep for an open pack of UHT milk and can I still let my girl consume it a few hours after opening it?

Depends on two factors: - level of contamination (bacterial) of the milk left in the container - time of non-refrigerated exposure If it is minimally contaminated and kept in the fridge, estimated shelf life is 5 days. Same is true (in different degrees) for all perishable foods including soy milk. The warmer it is, the more bacteria it will grow. In fact, you can prepare small packet for her to bring, it nay be better that way.