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?

Once you open an UHT milk, similar rules apply to pasteurized refrigerated milk, normally about a week in the fridge. For your daughter's day at school, I'd recommend small UHT packs that can stay closed until she drinks it. If that isn't an option, buy a bottle that keeps the milk cold or at least protects it from heat and light.