Depends on a state, since educational legislation is mostly a state province.
Predictably, California has an
app law for that:
What are the required instructional minutes for an elementary school maintaining any of grades one through eight?
If a school is identified as an elementary school in the County-District-School (CDS) code system, EC Section 23 states that, “Notwithstanding the provisions of Sections 51210 and 51222, instruction in physical education in an elementary school maintaining any of grades one to eight shall be for a total period of time of not less than 200 minutes each ten schooldays, exclusive of recesses and the lunch period." (src: http://www.cde.ca.gov/pd/ca/pe/physeducfaqs.asp)
CA Educational Code (http://law.onecle.com/california/education/index.html) lists several sections clearly dealing with PhysEd, most in 512xx range:
California Education Code Section 51210.1 (a) (1) The Legislature finds and declares all of the following: (A) The Education Code currently mandates 200 minutes of physical education every 10 schooldays...
California Education Code Section 51210.2 (a) The Legislature hereby finds and declares that the physical fitness and motor development of children in the public elementary schools is of equal importance...
California Education Code Section 51210.3 (a) The governing board of a school district may designate a credentialed teacher at each elementary school as a science coach, or provide staff development...
Same thing in New Jersey:
Are all students required to participate in the health and physical education program?
N.J.S.A. 18A:35-7 requires every pupil, except kindergarten pupils, attending public schools to take such courses. However, the core standards establish requirements for students in grades K-12. (src: http://www.state.nj.us/education/genfo/faq/faq_chpe.htm)
For other states, googling for "physical education mandatory law " yields good results based on the above 2 states.