Potty training for bowel movements

Hi, our daughter is 4 and has been at primary school for 4 weeks. She’s still not fully toilet trained when it comes to pooping. This has resulted in quite a few dirty diapers for a while now. We tried telling her that if she felt something, she should say so, and then she’d get a diaper. That worked for a while. Then we said we’d try the toilet first; if it didn’t work, she’d get a diaper when she felt the urge again. We eventually worked up to trying twice. Our girl eventually didn’t want to do this anymore and sometimes only wanted to sit on the toilet for a few seconds. To keep her on the toilet longer, we started chatting more actively, reading a story, or sometimes even watching a video on a phone or tablet with her. We noticed that when she was on the toilet, she was just sitting there and not actually trying to poop. Now that she’s at school, she’s having a relapse. She has dirty diapers more often (almost always) and doesn’t mention it anymore. Sometimes she says she doesn’t feel it when she’s pooped in her pants (which is hard to believe because she often clearly stands in a corner and strains), and other times she says she finds it stressful but doesn’t know why. At school, they (understandably) don’t have time to change her pants every time. Last week, she came to after-school care with a very dirty diaper, and once the teacher called us to pick her up. It gave us the feeling that she was being punished for pooping in her pants. The teacher said that was absolutely not the intention, but we think it might have happened unconsciously. Does anyone have tips? We’re at a loss.