In my opinion, self confidence cannot be taught. It's something that people gradually learn by themselves.
I recommend you to just set an example of confidence for her in your day-to-day life to influence her. You can also just try to be positive and encourage her often, such as complimenting her and not acting all disappointed if she gets a bad grade.

Basically, just be positive to her and try to influence her by being more confident in your own life since you can't directly make someone confident.