The answer is "it depends". If the decree says one thing, but the parties have, in practice, been doing something else, the judge will often modify the terms of the decree to match what's actually been happening. But, usually, that will only be done if the parties have been varying from the decree for an extended period of time. Your question makes it sound like your ex has only had a handful of extended weekends, which makes it less likely that the judge would feel compelled to modify the decree's terms. Additionally, the fact that the change your ex would be asking for has already created problems with the school is another good reason for the judge to refuse to modify the decree's terms. And, to put the shoe on the other foot, your ex might want to be careful of opening up a modification suit because he could lose the Thursday visits if he's not exercising them.
So, based only on what you've said, it seems like your ex may have a tough time getting what he's asking for. But, all the same, if a modification suit gets filed, you really need to have an attorney helping you handle it.
You're in a fight here. Generally, courts tend to let parents who want extended standard visitation get it. You can elect it under the Texas Family Code. However, you have to demonstrate that it would be in the best interest of the child. If he's trying to modify it and you don't feel it's in your child's best interest, you need to let the judge know that.
If he's bringing a suit, I'd strongly recommend consulting a local attorney.
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