Need advice about teenage girl and boy issues!

My daughter is a freshman in HS and barely 15, just turned last month.  This summer, while she was still 14, a senior boy who is 17, started "talking" to her which I understand to mean he liked her and wanted to start dating.  I said no.  He's too old.  She was all "he's nice and I'm almost 15" blah, blah, blah.  Her father and I are divorced and one weekend when she was with him, her father allowed this boy to take her to a movie on a Sunday afternoon without asking me.  The boy picked her up in his car and took to the movie and brought her home by 7 p.m.  I was upset, but decided since it seemed harmless, I wouldn't make a big deal or have a fight with my ex.  Long story short, my daughter snuck out in the middle of the night one night to ride around with this boy.  She was grounded and I told her she could not see this boy anymore.  I had also checked him out and found out from some other parents that he had a reputation as "partying" and there had been some incidences with pot.  I told me ex and we agreed this boy was history.  Well, the boy started dating another girl and when school started my daughter was liking a boy her own age.  I ungrounded her after a month and all was fine and dandy.  Until this weekend, my daughter and a friend wanted to go to a local amusement park and I dropped them off with instructions I would pick them up at a certain time that evening.  A few hours later I get a text from another mom who was at the same amusement park with her kids who said "did you know your daughter and her friend were meeting with older boys at the park?"  I said no and asked her who and one of the boys was of course this older boy who I thought had forgotten about my daughter and she about him!  Apparently not!  She's grounded again.  I've tried to talk to her reasonably about this, without yelling.  I asked her what was so special about this boy and she said he makes her feel special which probably means he wants to get into her pants and I told her that.  She rolls her eyes and tells me I don't get it.  I told her he was not the boy for her.  That he is a fast mover and parties and I don't want her around that.  She says I'm being judgmental.

So here is my question, what do I do?  She's obviously going to lie to me and sneak around to be with this boy.  What do I do short of locking her in her room and homeschooling her until she graduates?  Six months ago, I would have told you that my daughter has a happy, well adjusted girl with a head on her shoulders, makes good decisions, and I trust her to always do the right thing.  Now, she's lying to me, is secretive, sullen and snotty.

The boy's parents are divorced and he lives with his father who basically lets him do whatever he wants.  I have asked my ex to talk to the boy and his father, but I feel like that will go nowhere.