The rule to allow the functionality to drop N lowest or keep N highest grades in a category needs to be enforced in the category settings and there needs to be help that explains when and how these options can be used.

See https://tracker.moodle.org/browse/MDL-46576 for the code to branch from, unless that issue has already been integrated.

See the spec at https://docs.moodle.org/dev/Natural_weighting

See https://github.com/MoodleGradebookWorkingGroup/moodle/compare/master...28_natural_weights for the original prototype implementation.

Weights need to take into account that some grade items will be dropped. This needs to reflected in the UI somehow. For example, maybe the weight would be "25% or dropped"