My concern is that you say you went to court for this, but have no ‘court ordered’ custody papers signed and filed by the court. If you just have a letter, I don’t see that it’s court ordered by the judge.
If your X DOES have a previous court ordered custody arrangement signed and filed (you would have a copy), then HIS order stands until the new order is filed. Custody order is formal paperwork issued by the court and signed/filed with date and a “clerk of court” seal or something to that effect.
Maybe I am not understanding your note, but unless you have a ‘court ordered’ document stating your custody arrangement in hand (and it doesn’t sound like you do), then his order is still valid. Once you get that order, I would keep it with you as evidence to any cop or person that tries to deny you access. I would think you could go to the school and talk to them too. I question why your X is SO touchy about you seeing your kids. You would think he would want an amicable situation for their sake.