I think that teachers have a range of about two grades that fit them best. For example, the best two grades for you may be fifth and sixth. Or they may be 11th and 12th. Every teacher is different.
I am not saying that these are the only two grades that you can teach successfully. My point is that these are your two best grades to teach.
If your goal is to be the best teacher that you can possibly be, then pay attention to what grades you are teaching. You may not always have a choice. If you do, though, do your best to get into your sweet spot. Your students will be rewarded.