First of all, the following questions are just for you, not for you to post to us. Who is listed in the child custody decree as the MANAGING CONSERVATOR and who pays child support? Who do the children ACTUALLY live with? My husband's ex was the managing conservator, meaning that the children were to live with her (they did) if we had to go "by the letter" and my husband paid child support. The answers to these questions make all of the difference in answering your original questions.
If he's the managing conservator, the kids live with him, and the grandparents are who he has chosen to pick them up, you don't have much choice except to go up there and have your name put in as "Mother's contact info" and to be able to see your kids' files.
If you're the managing conservator, the kids live with you, and you have chosen to allow the grandparents to pick them up, then you need to take control of the school paperwork by going to the school every so often and checking on it, getting to know the faculty, and letting them know you're involved. If the school only sees the Dad and grandparents, it's easy for the school to assume that YOU are the weekend parent. Most schools have websites now, but you have to be in the paperwork as Parent to have web access to the kids' files. You can use email to contact the teachers to set up parent-teacher conferences and to stay up-to-date with the teacher. If you can't work with the Dad to go to conferences together, then DEFINITELY have conferences individually. The teacher would prefer to work with y'all together so that you're "on the same page", but they know it's not possible in many families nowadays, and will work around that for the best interest of the kids.
We did have a couple of these issues a few times: once our son was hospitallized and we didn't find out until a month later because our daughter asked why we didn't go see him. She was 15 at the time with her own cellphone, so we asked why she didn't call us? She said she "thought about it but forgot". Their Mom would store up out-of-pocket medical receipts not covered by insurance until she had a large monetary amount for us to pay and then give them to us. She would then threaten us with a lawyer if we didn't pay within a week.
While we picked the kids up from school everyday (the school knew me just as well as they did Mom), my husband was listed on the paper work as Father and had his contact info listed and I was simply listed as an emergency contact and the "normal way home". When paperwork for updating contact info came home, we would fill in our info and send it on home with them. Once they got to high school I would park in the lot and pick her up from school. The Mom would fill out all paperwork and we found out the hardway that we weren't even emergency contacts. I went to drop off current copies of the medical insurance and dental insurance cards to our 17 year old and was told they could not deliver them to her because I was not on the "approved contact list". I asked why they couldn't just deliver the cards with no note from us and they said it was to prevent parents without rights from having contact with the kids as well as preventing "stranger danger". I explained that their mother asked for them and that I was also the one who picked her up everyday after school. They explained that because I wasn't on the list, I wasn't supposed to that! I asked them to call my husband to get permission. They said he's not on the "list" either, so they can't do that. They wouldn't even call the Mom for me even though I knew her number by heart and it matched the one on file (they didn't tell me that, but I had just talked to her 30 minutes before this). I had to drive over to the grandparents' house to drop them off. Everytime after that if she called for a during-school appointment, I would ask "have you put us on the list, yet?" She'd say no, so I'd have to tell her to have her aunt (who worked) pick her up and drop her off to me. I told my husband that he needed to take his custody papers up there and fix it, but he didn't want to rock the boat. I am the one who stays home and my husband's job is literally across the street from the high school while Mom's is about 20 minutes away. If something serious had happened during school hours, we wouldn't have been able to even talk to her, according to the school rules. We would have had to meet her at the hospital and then take care of her. The "kids" are both in college now, so those worries (the "not being able to help because of school rules" worries) are gone now.
Hope this helps. Good Luck!