The origin of this goal is the effort to find creative solutions to the STEM crisis. While teaching waivers are not typically used this way, teaching waivers could
be used to allow competent professionals from outside of education to be hired as part-time instructors to teach courses such as Advanced Placement chemistry or calculus as long as the instructor demonstrates content knowledge on a rigorous test. See NCTQ, "Tackling the STEM Crisis" at: http://www.nctq.org/p/docs/nctq_nmsi_stem_initiative.pdf
For the importance of teachers' general academic ability, see R. Ferguson, "Paying for Public Education: New Evidence on How and Why Money Matters," Harvard Journal on Legislation
28 (1991), 465-498.
For more on math and science content knowledge, see D. Monk and J.R. King, "Subject Area Preparation of Secondary Mathematics and Science Teachers and Student Achievement," Economics of Education Review
12, no. 2 (1994), 125-145; R. Murnane, "Understanding the Sources of Teaching Competence: Choices, Skills, and the Limits of Training," Teachers College Record
84, no. 3 (1983)