Testing vs. Attendance

Recently I took several AP and IB tests. For those who do not know what the acronyms mean, they stand for Advanced Placement and International Baccalaureate tests, and they function as advanced tests that account for college credit or placement if a high enough score is obtained. They are the culminations of a year or two of advanced high school classes, and test a student's ability to master advanced material in a variety of fields.
I can attest that these tests are very difficult, depending on the person's abilities. For instance, in my case, the mathematics IB and chemistry IB tests I took were difficult, while the English and history exams were rather less difficult. But whatever the level of difficulty, these tests adequately measure a student's knowledge and skill in the given area.
While I take an IB or AP exam, I am excused for school with my absence not counting against me. I am responsible for obtaining whatever homework is assigned while I am gone, and completing all assignments as quickly as possible.
Two odd things occurred during this period that prompted offshoots of revolutionary thought within the attention-deficit corridors of my mind. The first was the realization that the free time spent outside of class actually forced me to learn more than I would in class, due to the pressure of the exams and their difficult nature. Also, without a teacher to rely on, I had to practice concepts over and over to ensure mastery. The second was that the little time I did spend in class was far more focused than the usual 59 minutes in a given period that I only spend 20% of working. Shorter class time made it crucial that I obtain whatever information I needed as quickly as possible.
These two observations sparked an interesting idea.
What if the public-school system began to shift its focus from attendance to testing?
To explain this notion that the public-school system focuses more on attendance than testing, I have to diverge a little from the main topic and relate some personal history. I was homeschooled until I was 15, learning music, English, and history from my mother, and Latin, mathematics and biology from tutors. When I determined to attend public school rather than a local community college, I attempted to earn some high school credit by taking a California placement test that measures high school aptitude. I scored in the 97th percentile, and attempted to present that score and documentation to my local high school.
Unfortunately, this did not account for any credit. The reasons given were that it did not demonstrate enough attendance in class that argued mastery of material, I could not prove I had mastered all the material or that I could perform well over an entire school year in a class, and also that the difficulty of public school differed vastly from homeschooling.
Essentially, my counselor told me, the time spent in a classroom was integral to ensuring a student had actually learned and absorbed the material, whereas a test cannot accurately measure an entire breadth of knowledge.
I realized an inherent contradiction: if AP and IB exams can measure college credit, how does a high school aptitude test differ from accounting as credit?
However, I simply decided to graduate high school in three years by taking extra classes at a local community college and obtaining some waivers. I managed to obtain the necessary credits after some extra effort during my senior and sophomore years as well as participating in several sports in order to obtain PE credits.
Thus, after laboring rather harder than normal to graduate high school, I reflected upon the choices I'd made, and returned to that original counseling conversation during sophomore year. And I thought, Why is it not permissible to focus more on testing overall during school rather than attendance as a measurement of ability?
Testing does accurately measure knowledge of a subject, as evidenced by the AP and IB exams. Other tests also are guaranteed by almost all college institutions to measure academic ability; SAT subject tests, GREs, MCATs, etc. Testing is used as a substitute for actual class time.
It does not fully seem reasonable that such a method cannot be used for high school credit. Perhaps one argument would be that such basic skills have to be learned through simple measurement of time spent in learning. Or another could be that not every kid who deserves a high school education can test well, thus they would not measure up to full standard in such a method even though otherwise they would function excellently in school.
However, I propose a compromise. Either a kid could account for credit by the usual method of attending class the full nine months, or he/she could obtain credit by testing out of classes, and earn credit for demonstrating mastery. Those who did not wish to test could attend school normally, and those who did not, could test out of their usual level. The key change would be that high school credit would be obtained by such a procedure, instead of mere placement. State laws would have to be transformed in accordance with new national regulations so that kids would be able to take standardized tests in order to move on to different levels in a variety of subjects.
What would such a process offer? Firstly, those children who were intelligent enough could graduate high school faster than normal, and begin careers or college-level education without the draining requirement of local public school attendance requirements. Secondly, the schools that supposedly offer kids this option by allowing local community college courses to account for high school credit would no longer have to force kids to commute and take classes away from their peers to obtain this option. Thirdly, homeschooled children could transfer easily into public schools without the usual brouhaha of counseling difficulty. There are several other benefits, yet those are the main ones that come to mind.
One objection is that school regulations as such allow for proper social environments and invaluble peer education. I would answer this objection with a question of why artificial environments of children between the ages of 14-18 ruled over by a few adults is considered the optimal environment for social learning when previous generations spent a majority of their social formation living with all manner of ages and familial situations. Socially, it is far better to experience diversity than to spend a majority of a year with kids of a similar age and experience.
I would welcome comments and discussion upon this question as I am sure that I have missed some points or perhaps there are aspects I have not considered.