If your school offers only a BA, but the course requirements in physics and math are up to snuff, don't worry about the fact that it's called a BA. Harvard offers only BA degrees, after all.
In the US, there is no standardization about BS versus BA degrees. Some universities offer both, and the BS signifies a more rigorous curriculum in the major field. At some universities, it may signify only a difference in "general education" requirements outside the major. Smaller schools often offer only one or the other, and the choice may be merely a historical artifact.
What counts is the courses that you actually take, not the name of your degree.