Thank you for your advise. I think I might have given you some wrong
impressions. Please allow me to clarity:

It's not that I want to prevent my SD from going places, rather than I want
to deal with the deception. She is in fact quite busy most weeks. Two
nights a week she is out at one organized activity or another and as I said,
every second weekend she has some activity going on. She often has a friend
over on the weekend for sleepovers.

As for the mixed party......I just want to clarify that I have NO problem
with mixed parties at all. She has been told she can have them here too (so
far she hasn't). My problem was that we were not told about the party until
after she got to her friends' place and that the party was supervised by
parents we do not know. I do know the parents of her two closest friends.
We socialize together.

Someone else suggested that I call the parents home which I actually do.
That's how I found out one time that the parent wasn't there after we were
told that the parent would be there. My big issue is that nothing is as it
seems with my SD. She tends to use words like "I thought" or "that's what
'so and so' said". There is always information that is missing and I am now
in the habit of asking a gazillion questions whenever she wants to do
something (which I actually would prefer not to have to do by the way). Mom
tends to just say yes without asking anything.

The trouble is, lately, she is talking about quitting her organized
activities in order to free up her time to "hang" with friends. She doesn't
want to have to miss the "more fun" activities because she is committed to
something else. I've always been of the opinion that organized sports is
very good for a child and I am disappointed to hear her recent plan. This
stems from the fact that my SD had to miss a dance at school because she had
a basketball tournament to attend. We've been trying to teach her that
staying with something she joined was important and that ditching something
simply because something else sounds like more fun was not the thing to do.
I think her coach who volunteers her time to this team would agree. Mom
agrees with keeping her commitment this year but thinks it's no big deal to
quit the activities next year.

Her marks have taken a dive since Junior High too. She used to be in the
90's but now she is bringing home marks sometimes in the 60's. She tells us
it's because it's too hard but all of her lowest marks are all equally the
marked with the lowest marks in effort.

The way I see it is, she is primarily concerned with social activities only.
I realize that this is perfectly normal for a teenager but I would like to
have a healthy balance. She has ditched things she didn't want to do by
saying that it was happening on a weekend she was to go to her Dads' and she
"didn't want to disappoint him", only to later call him up and switch
weekends with him when a fun thing came up.

So am I supposed to just back off completely and let her go where ever she
wants, whenever she wants? Should I let her grades keep dropping and never
mind the comments on the reportcard regarding the lack of effort? I'm not
sure I can just sit here and watch that. I've watched other children grow
up doing pretty much whatever they want and they are not doing so great now
that their in their 20's. Not in school, working enough for money to party
with.....not paying room and board.....not really doing anything
constructive.

Up to this point the kids have been taught to be very responsible and so
far, they have been great at it. They do chores around the house without
any problems and are both quite capable of making a meal for themselves. I
think it has boosted their confidence greatly. Their mother has done a
great job with them and I like to think I had something to do with it too.
They really are good kids. I'm just worried that if we give my SD too much
freedom that she might go down the wrong path.....that's all. If we let her
quit her sports......I'm afraid of what will she might migrate to. I've
always thought that it was better to keep a kid busy in organized activities
than to have them have nothing to do.

Thank you for you responses.