Find the teacher who changed your year and email them. They almost never hear it.

Teachers mostly find out they mattered by accident, years later, if at all. The one who changed something for you probably has no idea which day did it, because on their side it was a Tuesday. Telling them costs four sentences and gives them something they keep. It is also practice at saying what somebody did while they can still hear it.

Name the specific thing instead of the general praise. The week they stayed after to explain it a third time lands harder than calling them a great teacher. Send it before it turns into something you meant to do.