Civil service codes vary from state to state in the United States. But even without any knowledge of a particular code, I think it is safe to say that distinguishing a pregnant teacher from any other teacher would be in violation of federal discrimination laws and the discrimination laws of most states. Treating a pregnant woman adversely because of her pregnancy is a form of sex discrimination, since only women can become pregnant, at least so far!
Notice, though, that pregnancy is considered a protected class under discrimination law because it is associated with the female sex, but it is not considered a disability under anti-discrimination statutes. It is a natural condition that is temporary, so it does not meet the definition of disability under federal law, and I would imagine, under most state laws.
This means that a school district must treat a pregnant woman as it would treat another person with a disability because otherwise, the school district is engaging in sex discrimination. I know this is a bit confusing, so I will give you an example.
Suppose a woman needs to take a month off because of difficulties in her pregnancy. Within the same school district, there is a male who needs to take a month off to have open-heart surgery. The school district approves the male's request and disapproves the female's request. This suggests that the school district is discriminating because there is no difference in the time off requested, yet the female's request has been denied.
This is a complex area of law, and would require a seminar to explain all of its nuances, but as a general rule, no one is permitted to make adverse employment decisions solely on the basis of a pregnancy.