This is so true. I realized this very thing myself in the second year in high school. I thought: what the hell am I doing trying to get good grades—from every subject?

I had been doing precisely that just because I could and with the eerie awareness of the fact that I would forget 95% of anything deemed uninteresting right after the course. When it came to subjects tangential to my interests, I realized that I was certainly not going to learn something in school that I couldn't learn by myself.

If I ever needed to cross check which European country invaded which other European country in the 1600's, how to translate "dear girl, I want to marry you for the rest of my life" to German should the opportunity arrive, or desperately need to exercise in some molecular calculations, then I was convinced that I would still have the chance to learn that stuff.

Except for the subjects of my interest, I then dropped all my effort to the very, very minimal level and as a result my grades dropped slightly. It was probably the 20% of effort, 80% of results scheme in action. In some subjects that I had decided to completely ignore, like religion courses (yes, we had those in Finland...), I let my grades drop to the minimum that was required to avoid flunking and was actually pretty proud of that result.

I graduated with good grades in everything I cared about. My high school diploma then exhibited real value for the period of a mere month until it claimed its purpose and I got accepted to the university. I've never needed it after that.

And, by the way, except for mathematics, 99% of what I learned in high school I have never ever needed since. I'm 32 now and I'm glad my teenage realization turned out to be correct.