Question

Your best friend's parents have refused permission for the 5 days outstation school trip.your friend hasn't shared the reason with you.

Answer 1

If your question is why your friend hasn't shared the reason with you, it might be because its a personal thing that her parents or even herself don't want her to tell people. Or its a reason that she might think that you think its silly and doesn't want to share with you.
Hope this helps! :D

Answer 2

It could be that your friend was embarrassed over what had occurred to make her parents not allow her to go.

Or that her parents didn't like the thought of their child being gone for so long.

Or even that she didn't want to go and instead used the excuse of her parents to soften the blow about not wanting to go on the trip.