^^This, what stepmomnorth said

If I were your DH, I would reply to BM and say he already has PTCs scheduled, so she can go on her own. No need to let her join the "me too" or "I agree" train, like our BM always did. Also, no need for them to go together. Teachers may grumble about having to do two conferences, but most are used to it, since so many students have divorced parents. DH finally asked for separate PTCs after he met me, because BM would tell him he "asked a bunch of stupid questions" when he went with her and would try to dominate the conversation to make herself look like a good mom. When he told BM he was going to have separate PTCs, she said to him, "now, you'll just ask a bunch of dumb questions and make us both look stupid!" I went with DH and she told him that he had no right to bring "some girl you just met" who was "not family" to PTCs, even though DH and I were getting married the following month and her live-in boyfriend (who she pretended to marry right after DH and I met, but didn't actually marry until two years later) had been going to the conferences for a couple of years at that point. He was allowed, in her mind, of course, because he was a "stay at home dad" and "does more for the kids than you ever will" (as she told DH). They are now divorced and she has a new boyfriend who is there all the time, but has his own house, so doesn't officially live with them. 

Our BM is similar in that the only things she does to help SSs with school is demand their teachers be changed if they perceive a teacher to be "mean" or if she has a tussle with a teacher. Other than that, she has no idea what they do at school. As evidenced when DH sent a long email including things he thought should be included in one kid's IEP because BM scheduled the meeting at a time that would have required him to take a full day off work and BM just replied, "I agree with all of this", probably without reading any of it. The one time DH did take a day off work to go to an IEP meeting, BM missed it, even though she was the one who scheduled it. Both kids also have IEPs that are horribly written, very non-specific. Also, BM once crowed to DH that their scores were "off the charts" and the teachers were "so impressed" with their improvement...because she didn't understand that the scores were actually saying both SSs had a 5th grade reading level, in the 8th grade. 

I'll add that I stopped going to PTCs after two years because as a stepparent you can't care more about education than the parents, so I had to back away from anything education related. Also, thanks to BM's histrionics with the guidance department at each of their schools, both SSs skate by with "adjusted" grades and don't have to do much work, but still want congratulations for their adjusted grades, and that makes me angry, so I really had to step away. 

Time to disengage. SD gets off at the wrong house - BM can pick her up or DBF can drop her off. School work not being done - not your problem. 

Well, I commend you on your dedication to this kiddo.

I too tried helping skiddo with her schoolwork her Freshman year (a year ago, starting the preceeding december so winter 2020 -spring 2021). I found out she was failing Art, Orchestra and Honors English, in addition to being behind in a few other subjects.

I was given full authority by my DH. SD15.5 B/M (this is when Munchkin got her other name Backstabber) also had given me full authority. I listed missing assingments, bought school supplies, zoom called teachers. When she pulled her grades up, I bought her a bed and an art workshop. so in addition to sacrificing weekends, I spent about $500. I coached and cajoled. Homework got done assignments got done.

Then, Spring break happend and so did Kansas City (a girl she talks to on the phone). The grades took a nosedive. When I approached her about missing assignments she was at her mothers house for that week. I threatened to take her phone away when she did not respond. I told her it was disrespectful. She finally sent back a very long text, to which I did not respond, accusing me of harrassing her, and "not helping at all". This led to a bunch of nasty texts from Toxic Troll who had not had a single shred of involvement. Basically SD15.5 B/M had shown her mother her texts to me, and from those believed her daughters accusations. Nothing was based on anything I had said or done, so she had to activate her mother based on her own texts. 

This led to threats of family court and full custody. Texts that took severe jabs at me. "Your going to lose Bakstabber/Munchkin just like you lost Feral Forger Sd22". All because I threatened the phone. SD15.5 spends 10 plus hours a day I had found, on the phone to this one person. I snooped because I was wondering about the sudden change in schoolwork and lack of responses. 

I disengaged from any kind of help from there on out. This was April 2021. 

I think you should focus that wonderful energy on your bio kiddo, ask your DBF to do a separate conference. Go together - you for your kiddo, his for his kiddo. Leave BM out of it and get your partner involved. He drives SD anywhere she needs to go. Protect yourself from the lies by not being available.