While it is extremely difficult, if not impossible, to make any viable argument pro or con when a premise includes the word only, there is much to be said about personal motivation. Certainly, it is the first and essential ingredient in personal success as many old sayings affirm (e.g."You can bring a horse to water, but you can't make him drink"). But, the fact remains that humans are social creatures and are affected by family, friends, lovers, and their environment. While, as the writer/philospher Johann Wolfgang van Goethe reflected, man is a herd animal who often does not wish to belong to the herd; nevertheless, he does, and his "herd" is often his greatest obstacle. For, there are interpersonal relationships and other social situations and obligations which interfere with an individual's goals. Is this not what Langston Hughes alludes to in his poem "A Dream Deferred"? And, does not the perennial favorite Christmas movie It's a Wonderful Life has as its subject the deferment of a dream because of family obligations?
Putting this point aside, self-motivation in the high school setting is, indeed, the initial key to success and the attainment of "the best possible high school education." But, as far as being the "only thing" that can detract from the attainment of the best possible high school education, statistics point otherwise. Consider, for example, the history of New Trier Township High School in Winnetka, Illinois. This school, founded in 1901 in a Chicago northshore community, was featured in Life magazine in 1950 and 1958 as one of the best schools in America. Since then, this school has remained in the top for SAT and ACT scores, National Merit Semi-finalists and finalists. In 2013, according to an article by the Department of Psychology at the University of Michigan,
New Trier students outperform their Illinois classmates on every conceivable measure.
Granted, New Trier spends much money per student, but there are many suburban high schools in Chicago that almost equal this amount, as well as other schools in New York or other large cities that have affluent suburbs. Since New Trier has remained at the top for over fifty years, there are obviously other factors besides the high motivation of students as individuals. Thus, in addition to one's innate intelligence, it can be assumed that a learning environment that fosters achievement and excellence is at play, an environment which includes competition among students and the direction of well-educated and competent teachers.
There is little doubt that highly competent teachers and a culture of competition improve students' successes. Students who are not afforded teachers who expose them to great works or great ideas often cannot reach the heights of intellectual achievement that others can. [The meaning of the word education denotes the act of leading outward]. This is not to say that an individual cannot overcome obstacles and rise to excellence on his/her own. Certainly, there are examples of those who have overcome poor schools and deprived environments, such as Dr. Ben Carson, a graduate of Yale and renowned neurosurgeon, who is famous for being first surgeon to successfully separate conjoined twins joined at the head. Dr. Carson attended inner-city Detroit schools and had only a poor mother to raise him; nonetheless, because his mother encouraged him and he did not stand in his own way, Dr. Carson achieved greatness. But, the path to success is assisted by support from family and from one's environment.
The idea that “the only thing that stands in the way of the best possible high school education is one’s self” is a harmful claim. Believing that a person can achieve anything if they put their minds to it leads to a false belief that if someone fails it must, in some way, be their own fault. This educational meritocracy prevents people from addressing very real problems such as racism, gender biases and socioeconomic disadvantages that all schools face. Accepting the idea that a student’s success is solely based on their work ethic, sets up the false argument of if student x can do it student y should be able to do it as well. This may not take into account circumstances outside of student y’s control that ultimately may have a negative impact on their ability to learn.
I don't believe that it is entirely up to the student (or one's self) to get the best possible education. After all, one goes to school to learn things they did not know before. Therefore, the student cannot know before hand what they are supposed to know to get a proper education. It is up to the teachers, faculty, and administration to ensure that the teachers are teaching the proper material in a way that students understand it so they can learn the most they possibly can.
However, the student obviously has a big responsibility as well. The teacher can talk all day long, but if the student isn't engaged or trying to learn, that it is pointless for the teacher to be speaking. It is also up to the student to go ask for help from a teacher, other student, etc, if they are struggling or don't understand the material. So it is mostly up to one's self to get the best possible education, but, everyone needs help and guidance to get there.
Hope this helps!