Take all electronics away. Ground her. Schedule a meeting with the school with the three of you and all of her teachers present. Turn this negative into a positive "we are all going to work very hard to get you back on track". She will need to earn back everything that you have taken away by going to school every day on time and bringing up her grades.
You may also want to schedule a complete physical with your doctor for her just to make sure that there are issues there.
I would also recommend that maybe she should have someone to talk to (a professional).
There maybe issues going on with her that you don't know about.
You and your wife need to work as a team.
I went through this with my oldest daughter and we had a team meeting at home that included her grandparents. Came to find out that she stopped doing the work because she had a learning disability and needed medication-who knew? She was all about her social life, had poor grades, was skipping etc and it was all because she felt stupid that she couldn't keep up in school but never said a anything.
Good luck to you!

Take charge--tell her to get her GED and get a job.

Load the Skid up in the car and take her to the nearest homeless camp. Inform her that she either graduates on time or rather than the family going to her graduation she will get dropped off to start her life with her new neighbors at the homeless/panhandler camp.

This scared the shit out of my SS enough that he pulled his head out and graduated on time and even with honors after he bombed the first semester of his Sr. year when the Sperm Idiot hacked the Military School firewall and SS stayed up all night every night playing WoW with his useless POS sperm donor. We jerked him out of boarding school and brought him home to finish his Sr.year at our local HS where we had moved a month after he left for Military School. He knew no one. That one was miserable kid for his last semester of HS.

Take her electronic life and flush it. Don't just take it for a while. Destroy it and if she wants it back she can buy it on her own when she launches. She gets no life until she steps up and performs adequately enough to graduate. Then if she does step up introduce her to the USMC recruiter and let her know that she has earned access to the home only until graduation at which point her key will no longer work.

The parents need to gain clarity that their job is to raise their spawn to be viable adults. A parent's job is not to coddle, or baby, or be a buddy to their children.

When my bride gained this clarity she scared the shit out of the Skid so badly that he came to me to ask for me to reassume the disciplinarian role. I tolerate no bullshit and I apply consequences as immediately as possible and taylor them to the offense. Once the consequence is applied I go into lather rinse repeat mode until the Skid learned that his only choice was to comply or suffer.

When his mom stepped up and took over as the primary disciplinarian she would not let anything drop and would ride his ass constantly on the issue at hand often far beyone the point where I would have closed with him that he had resolved the problem. He knew that if he and I had to readdress the problem it would be exponentially worse for him but at least with me there was an established expectation and process he could follow to get out of the shithouse.

With his mom ...... it was life with Medussa for a very, very, long time and an extended residency in the shithouse. }:)

Lot of good ideas in this thread but not a one of them will work without full teamwork from your wife. Indeed, she has to take the leadership on this.

At my house we discuss behind closed doors between dh and me and then all sd sees is dad coming down on her. Only about 5% of the time do I speak up directly in front of her.

The child knows who her parent is. If she is not accepting the authority of other adults at school, why on earth would she accept yours? The only one who can make her do anything is her own dang parent.

If you step up as "disciplinarian" you are doomed to fail. Being proper parenting partners with your wife you can take a more active role (I do at my house cuz I know dh will back me up; even so, in the case of the sd, I rarely do) but without that you are just stepping into the Ninth Ring of Hell.

BTW, my sd was doing all these things, too. Freshman and sophomore year. Put us through hell and bm enabled all of it. SD seems to be doing better as a junior. DH singlehandedly thwarted her descent into high school dropout. It helped motivate him that I spoke up loudly that no high school dropouts were welcome in my home.

Quite honestly, my advice is to tell your wife that you support any decisions she makes and will stand by her side, BUT she must be the one to draw the line. Her daughter will not respond in a sane way to you being the disciplinarian. In fact, it may make a bad situation worse.

Follow your gut. Your wife wants you to be the bad guy, but she has no intention of following through with whatever you law you lay down. In fact, I guarantee that she will back up her kid instead.

Be supportive but as the step-parent, you can't do more for your skid than what her bio-parents are doing for her. Anything you do will be violently opposed by the skid and will not be supported by your wife.

At first I thought you were the stepdad in my case!!! AKA the Girhippo's husband. And I thought "it's about TIME that poor man cracked." But then I read the part about your wife being tough on the boys. The Gir is NOT tough on ANY of her children!

If the kid is old enough send her to Job Corps.