The first place to look for answers is in the parenting plan you established when you divorced. You should carefully review the parenting plan and make sure that education issues consist of joint decision-making. Often, one parent will provide the other parent with school notices, including progress reports. If school issues are decided jointly, you should able to have access to these materials.
Unfortunately, unhelpful principals complicate matters. Provided that you have a right to the records, schools should be willing to work with all parents to provide materials. You might consider writing the principal a letter requesting the documents you seek, taking action only after the principal refuses, in writing or by ignoring you, to provide the materials. You could always escalate to a superintendent of schools.
As for your former spouse, if she is supposed to provide you with materials under the parenting plan, you should also look at the section of the parenting plan as to how you are supposed to resolve disputes prior to going to court. This might involve mediation. Of course, if all else fails, you could go to court and ask the court to enforce the provisions, which might mean asking the court to order your former spouse to provide school materials.
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