It’s hard isn’t it? As lawyers, we don’t always know the gender of the person we are writing about or we are writing about abstract issues that could apply to anyone—male or female. His sounds sexist and it is inaccurate where you don’t know the gender. Her sounds a little too political. His or Her is more accurate than either word alone, but it still sounds awkward and technical.
You can avoid the gender minefield by using plurals. Rewrite A student may leave his or her books on the tables as Students may leave their books on the tables.
And never use his/her. His/her fails the test for plain English because it is not even pronounceable.
In a longer work, such as a book, you can always turn to the marvelous Dr. Spock for an answer. In his classic book, Baby and Child Care, Dr. Spock talked about his abstract babies by alternating between his and her. It worked in 1946 and it is still a good technique. (Dr. Spock introduced each abstract baby, by saying “Let’s say it’s a boy (or a girl).” That introduction that would not be necessary today, since we are accustomed to thinking across genders.)