As primary parent what rights do I have as to what type of schooling my children will have?
I have primary custody of my 3 year-old twin daughters. I want to home school them until 3rd grade my mother has an education background). My ex-husband wants them in public school. What rights do I have?
Primary custody refers to who has physical custody over the children. The primary parent doesn't necessarily make all major decisions concerning them.
What you failed to mention is whether or not you have joint legal custody or sole custody. With joint legal custody, both parents retain the right to make important long-term decisions affecting their children's welfare, including those regarding education. In a sole custody situation, the custodial parent is the one who has the full right and responsibility to makesuch major decisions.
Therefore, the answer to your question depends on just what type of custody was granted. You need to review your court documents to determine this. What I can tell you is that if you have sole custody, legally you can make the decision to home-school your children.
Note: If you have joint legal custody and you and your husband cannot agree on this, you will have to go back to court (possibly mediation, whatever your custody agreement provides) to get this matter settled.