How frustrated you must be. Unfortunately, and I know this from experience *, your husband is the only one who can arrange with the teacher for you to be involved. Being married to him does not give you any rights with his daughter, your step-daughter.
You might be able to arrange for a conference with you and your husband and the teacher. But be sure that your husband attends and backs you up. If you have the conference and he doesn't firmly state that he wants you involved you're even more lost.
I also have a suggestion that might be a little bit helpful in your relationship with your step-daughters' birth mother. I am responding from the position of being an adoptive mother. Your husband's ex no doubt is a difficult woman to deal with and I hope that your husband is the one doing the dealing. She is his ex and therefore his responsibility. She is no doubt still angry that you took her place. Because she is not willing to be co-operative it is very important that you not insert yourself into their relationship. It is also important to acknowledge to her that you recognize that you are the step-mother and she is the mother. Doing both of these things will help to bring down the feelings that are becoming increasingly more angry. Try to see this situation from her viewpoint. If you could acknowledge and accept her feelings she will be better able to be co-operative.
I am glad that you consider her daughters also your daughters. They are your daughters in many ways and thinking of them as daughters may enable you to feel closer to them and nurture them.
Acknowledging the difference between birth mother and adoptive mother was difficult for me while I was caring for my perspective adoptive daughter as a foster mother. My daughter's birth mother insisted that she was the mother and I the interloper. And legally that was true. Fortunately, Children's Services Division had the actual legal responsibility. With difficulty I accepted that Michelle was her mother and that CSD made all the important decisions. CSD suggested that Michelle became Mom Michelle and I was Mom M.. This helped a lot. Each of us had a role in our daughter's life even after her parental rights were terminated. She could no longer see my daughter but she still was important to my daughter as her birth mother. I remained Mom M. for sometime after the adoption and my daughter continued to call her birth mother, Mom. Eventually she dropped the M. but only after she felt that I was her mom. Once she became 18 she did begin to have contact with her birth mother and once more I felt jealousy because I was the one who had taken care of her all of those years and my daughter was once identifying Michelle as her mom. This was a difficult time for all of us. Eventually I was able to really feel that calling Michelle, Mom, did not mean that my daughter loved me any less. I was not in competition for her love.
I still feel jealous at times but I am now confident that what I do for my daughter is what is most important and a part of what I do is keep peace with Michelle. I still do what I think is most important for my daughter even if it upsets Michelle but I refrain from responding to her anger with anger. Confidence and the law allow me to do this. It's not such an easy distinction for you.
I think it is important for you to continue to support your step-daughter's education by remaining in contact with the school. Perhaps in order to do that you will have to make some compromises. I wonder about continuing to go with your step-daughter to school and picking her up. I think that once you had helped her establish a pattern of success you could back off and see what happens. She may be able to continue without your presence. And that is true success. If she is only doing well because you're forcing it she will stop doing well when you stop. And you cannot always go to school with her. Your goal is to teach her the importance of meeting the requirements of school so that she can do it because she wants to do it.
I suspect that at least part of the reason that her mother has banned you from going to the school is that her daughter has complained about being embarrassed. She may also see this as a way to get back at you for trying to take over her role. When you call yourself her mother instead of her step-mother that is what you are doing. She has a role as the mother and has the right to parent in the way she wishes even if you don't agree with what she is doing. You have the role of step-mother and you have the right to parent in the way that her father allows you to parent. But you have no rights that the mother or the school has to acknowledge. Only the birth mother and birth father has rights.
Have you read any about being a step parent? If not I recommend that you find some books that deal with blended families and step-parenting. Professionals say that step-parenting works best when the birth parent (in your case her father) has the primary responsibility for the child. This is maybe not possible when the birth parent abdicates his role which seems to be the case here. That makes successful parenting even more difficult.
Another resource for you is mediation. This may be available thru the court system. Or you may be able to obtain it privately with your health insurance paying for it.
I also recommend counseling so that you and your husband can come to an agreement acceptable to both of you of how issues are dealt with with the ex-spouse. As well as learn how to best help the children caught in the middle.
I'd also like to add a word about how your step-daughter must feel caught in the middle and suggest that this may be one reason that she is not succeeding in school. It appears that you and her birth mother are fighting over who has the most right instead of focusing on how can we either work together or at the very least accept each other to make sure that our shared daughter/step daughter has the most benefit. You go to school everyday with your step-daughter. Her mother doesn't and doesn't want you to either. You're at an angry stalemate. How do you get past that? If you remain there your step-daughter will be the one to suffer most. Having to work to get along in both homes will become more important than getting along in school. When each mother bad mouths the other the child is hurt.
Being a step-parent is even more difficult than being a parent and that is the most difficult job in the world. I wish you success and hope that you will find a way to make this relationship work.
*I was a step-mother and I've also had to professionally intervene in families with step-children and step-parents. I was not successful as a step-parent because my step-son's father was a passive parent who did not have rules or consequences and did not want me have any either.
I have seen other families succeed in building a co-operative if not friendly relationship thru mediation and counseling.