Late summer ball

I've got a summer ball coming up with my boyfriend at his old high school. I am allowed to go and I've bought everything for it. Only thing is I've just been told that his dad can't drop me off as it ends at 12pm so he said I can stay the night at his. I am worried to ask my mum as my parents are very strict which I like as they care but it gets a little to much as times as I want some freedom. Im just going to say to her he is sleeping downstairs and I'm in the guest bedroom. But how can I tell her to allow me to stay over the night!?! Help please!!