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 how you store the milk after opening the seal. If milk carton/pack is opened and kept in the fridge, you will get more than 5 days shelf life out of it. Therefore, it should be safe to give your daughter the milk if its from the fridge. Best to get milk in cartons with caps instead of those you cut open, so you keep minimal contamination after opening and re-storing it. If you are buying individual packs (those with the straw), best to only open it when your daughter is ready to drink it. In Singapore's humid weather, best to avoid exposing food items out in the open for too long. Bacteria tends to grow and thrive in warmer temperatures.