They can listen to the request of your guardian but they don't have to do what your guardian says. Assignments are done for certain reason. If you have a legitimate excuse to present after school than maybe your teacher will allow it but it has to be a pretty good reason because there are so many other things that teachers are in charge of doing. It's not fair for you to assume that they should take time from after school preparations to listen to something that everybody else probably has to do during class.Can a parent excuse me from a school assignment?
I think it depends on how good the reason is. If it's medical, then most likely. If not, then pray your teacher will listen to your guardian.