That's rough. Don't know if this advice comes too late or not, but I agree. Be responsible, don't pester her about it, and then make your case while she is in a good mood, and has time to listen. Reassure her that it is only for a day or two, there will be only (2? 3?) people there. Tell her there will be no alcohol and no "monkey business" and that others may have marijuana with them but that you will be responsible and abstain. Ask her to go out just for the day, if she's leery about you spending the night there.

And above all, you have to respect her wishes, even if it's unfair. You're not 18 and she's still in charge.

I had very strict parents growing up and I know this would not have flown with them, either, so I understand where you're coming from. But I think this technique is the best; you are telling the truth, and asking for her trust, but you are willing to compromise if necessary.

It's the mature thing to do.