A hypothesis is a statement regarding a possible outcome or relationship that can be substantiated by using empirical evidence. An important initial step is establishing an appropriate scale. Given that there are millions of high school students, additional parameters are needed. Will you examine the impact only on pregnant girls or will you consider the fathers as well? Are you concerned with pregnancy at all stages, only with those carried to full term, or those ended through miscarriage or abortion? What is your time frame—years, decades?
Another set of factors to consider is the available evidence. What studies will you consult to support the position you take? Available data will likely range from a school district up to national level.
Once these variables are selected, it will be possible to identify and consult appropriate studies. The evidence will help you decide if a hypothesis is positively or negatively supported. For example: In California, girls who become pregnant are more likely to finish school if this occurs in their senior year.