"But when I try to not think about it, I can't not think about it!" - astute 6th grader.
This truth my friend pointed to actually has a name. It's called, "the polar bear effect." If you try not to think of a polar bear, for example, some part of your mind has to keep thinking about it to keep you NOT thinking about it.
One of the biggest misconceptions about mindfulness is that we are trying to force thoughts away. Clear the mind completely. Instead, it may be more helpful to understand the practice as creating space so we can watch our thoughts get kicked up and settle. We may be swept away in the windstorm for a moment (or hours), but eventually we will open back up and see that it just happened.
This week, perhaps play with noticing when you are "in" the thought and when you can see that it happened. Rather than fighting against having the thought (doesn't work), start noticing it as one of other thoughts, feelings, sensations in the world. Give it slightly less power.