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?

The same rules apply to a pack of UHT milk as regular pasteurised milk, when you open it. However, for your daughter's school milk intake, I would suggest you giver her the small packs of UHT milk. All companies have them, and include chocolate and strawberry flavours. I tend to chill the UHT milk carton before sending it with my daughter in an insulated snack bag, so that it can stay cool.