This may sound a little fruity. I’m going out on a limb here and gathering information that adults describe as being helpful, which could also be safe for a kid to try to improve their focus and mood.
Prayer - ask God for help and insight about how to focus and learn, how to enjoy the process, and whatever else one can’t seem to do easily.
Gratitude Journaling - Writing down the good things makes them more noticeable, and over time changes the way one views the world. The effects persist even months after regular journaling has stopped.
Thinking about other people - we get in a rut when we think about ourselves too much. It’s refreshing to have a bigger purpose or remember that another person needs help.
Positive language around learning - If a student in my class says “I’m not good at math”, I stop them and tell them that categorical statement isn’t doing them any favors. “I’m getting better at math because I’m practicing” is a better statement.
“Meditation” - any technique of even breathing and directed thought, especially with eyes closed, changes brain waves and calms the nervous system
Stand up (or sit up) Straight with Shoulders Back - better for breathing and your back, shows confidence.
Listening to classical music, especially with eyes closed - when I was pregnant with my fourth kid I participated in a study which involved listening to 30 minutes of certain classical music every day. While I think the study was intending to evaluate the effect on mood, and it did seem to give me a boost, the most noticeable effect by far was that I suddenly found myself FINISHING THINGS instead of starting lots of things and suddenly looking around to realize I hadn’t finished them. It was a big change from the usual.
Cold Showers (or alternating hot and cold) - Not surprisingly, this changes circulation to the skin and increases alertness. More interesting, it improves the immune system and relieves stress and depression.
High Intensity Exercise - briefly pushing your limits (30-90 seconds at a time with rest intervals, for just 10-20 minutes) sets off a cascade of events that change metabolism for hours afterwards. Parents have a sense for which of their children need more exercise.
Finally, consider sharing with your child that in the real world, A students often work for B (or C or D) students. School is harder for some people, and the discipline one develops from applying oneself when it’s not easy can be a huge blessing later. (Take it from the A student who is still working on even a modicum of self-discipline 20 years after high school.)