Unfortunately.. it is your DH's place to discuss his daughter's schooling with her mother.. not yours.  Why won't he address it?  It takes TWO to fight.. he needs to be the one to deal with it.  Now, it may be normal for the girl to have some adjustment issues.. but I am wondering if the school might be able to help with that.. maybe he can talk to the school?  Unfortunately, your opinion is likely to not carry much weight with BM and may not be a welcome intrusion.. the school is most likely not going to discuss anything with a non-parent.

BD needs to take the wheel if he is concerned. He can talk to the school about his child's adjustment to the new school. Under FERPA  a non-custodial parent, who may not have been awarded any custodial rights, but who has visitation rights with their child, is entitled to full access to their child's educational records. He can go to the school and talk to the teachers about his concerns.

Whether or not BM can change the school unilaterally depends on the CO.. Changing schools is not always spelled out in orders. If there is no specific language requiring DH's consent, BM probably can decide.

Uniforms are not unique to Catholic school. The local public schools here have them. Not being Catholic and attending Catholic school is not unique either. Here, many non Catholic children attend Catholic schools. The Catholic schools here have accommodations for non Catholic students. When DH meets with the teacher, he should reinforce the fact that DH and his daughter are not Catholic. Non Catholic attending children attend and observe, but do not participate in services.

 Don't let your husband hide behind your skirts. He needs to be the one to deal with HIS ex and issues pertaining to THEIR child. 

I'm curious about who is paying for private school. Is BM footing the tuition bill? Regardless, it's important that your DH contact her in writing to document his concerns with BM changing their daughter's school without consulting him. He needs to specifically state that he doesnt agree with the change, list how it's affecting their daughter, and state that he will not be paying for it. Otherwise BM might try to stick him with part of the fees down the line.

Are you sure it's all the school?  It sounds like SD has had a lot of change lately, between the new school and BM remarrying.

I agree with others who've said that BD needs to deal with this.  If he can't talk to BM without fighting, he should request a conference with the teacher and see if he can learn anything more about what's going on with SD.

MalPal, you mention Mom has residential custody. What about legal custody?

Does Mom have sole legal custody (meaning Mom has sole decision making power in items such as education, aka which school she attends, medical, ect.)?

Seems strange you are the one approaching the mother over school rather than the child's father. Even if the parents have a difficult time communicating in a civil discussion, it is still his place/role to be the co-parent when it comes to the child. 

And yes, if Mom recently remarried (a stepfather now in the picture) and the little girl also changed schools (different routine from what she was participating in, loss of friends at her previous school blah blah) these are all considered major adjustments in a young child's life.

How long go was it that you came into this little girl's life? How long was it from Mom and Dad's divorce and her living with both Mom and Dad until you moved in and married the father? Didn't the child have somewhat of an adjustment struggle at that time of her young life? 

We've heard what Mom thinks of what's happening and we've heard what you think of it.... but what does Dad think of what is going on and why isn't he speaking up and for himself? Even if the parents don't get along there are communication methods for parents manage to work together whether that be non-verbal non-face to face program such as Our Family Wizard or through lawyers and/or mediation. 

Before my DH got custody of my SD back in 2002, she was placed in a private Catholic school.  BM at the time had no religious belief, but her boyfriend was LDS.  SD was put into the school because it recieved no federal or state funding, thus limiting DH's access to anything related to SD.  The school was not obligated to tell him anything until we started digging.

There are two kind of bm's

1. Who take courts seriously. Who value their x as an equal bio parent, working together for the good of child.

2. Unilaterally make all decisions. Maybe informing x after the fact or not at all. 

If you have #2 bm the road will be long without much help from courts. 

Wait until dh discovers bm's new hubby is listed as dad on school papers or next of kin for child. 

Your dh must make his presence known to everyone and that includes calling bm's local cps office to let them know bio dad is actually alive well and available if bm is arrested for anything. #2bm have a habit of forgetting bio dad is alive but they sure do like being bankrolled by them.

Catholic School is a great education.

Goodluck 

There is likely not much you can do about this.  We put my SS in Military boarding school his Jr. and Sr. years of HS.

SpermGrandHag went nuts but ... we just told her to FO and mind her own business.  The SpermClan kept paying CS but other than that they bore no financial responsibility for paying for boarding school for SS.  Once we did it and SpermGrandHag learned that 1. there would be no impact on them financially and 2. SS would not have to join the Military she STFU and crawled back under her rock.  SS thrived at that school.... until...... but that is a different part of the story.

The SpermIdiot didn't care one way or the other since he never paid a penny of his CS obligation for SS (SpermGrandHag and SpermGrandPa  paid it for him) and rarely saw the Skid even when SS was in SpermLand for his COd visitation.  SS spent 99.9+% of his visitation time with the SpermGrandParents.

Anyway, if BM and her new husband are paying for it.... give it some time.  It may provide a better education for SD. 

Also, SD may be picking up on your tensions and the tensions of her father regarding your feelings about her attending the new school.