Tonight your child will be bringing home their Chapter 10 social studies test to be signed. Should it be 'their' or 'his/her"

It should be worded... I get this weekly from my son's teacher...

"Tonight your child will be bringing home THE Chapter 10 social studies test to be signed and returned."

Although the answer with the word "THE" sounds right, I always thought that using "his" is the proper way to refer to a person whose gender is not known. You could use the word "her," if you are into women's lib. Using "his/her" gets old after while, especially if you use the term several times in one paragraph, or even several times in the same term paper.