Do your homework, and you will get a good grade. Don't do your homework, don't complain about your bad grade.
It's really that simple, you don't need to be smart for calculations at all. All you need to do is put in the practice. It takes no intelligence.
The homework will run you through pretty much every scenario you can expect to run into on a test. Do your homework, and you're prepared. If you get some wrong, just compare your work to the answers. If you don't understand the theory at least memorize the steps and you'll get it right and you'll be fine, the understanding will probably follow after enough practice.
You won't need to actually understand mathematical concepts until college classes ask you to apply math to find a solution instead of just giving you a problem and asking you to calculate an answer. If you're not planning on a math-centered career, then all you need to know is the calculation. And even if you're not going into a math-centered career, you still need to at least know algebra FFS. Even graphs.
Even a landscaper needs to know how much profit he's actually making so he can figure out what he should charge. /EVERYBODY/ needs to understand how interest rates work so that they can figure out what kind of mortgage they can support and how much savings is necessary to retire someday.
People today from ages 20-30 probably won't get any help from social security when they're at retirement age. The only way they can retire is if they prepare in advance. If they don't understand compound interest, they will be 75years old and still working at walmart just so they can have healthcare. Compound interest is so powerful that you can invest 8k by the end of highschool (that's 2k per highschool summer job) and retire as a millionaire! (Just dump 8k into a Roth IRA account, toss it into a index fund and withdraw tax-free at 65). The earlier you save, the less you have to save because the interest will work for you. Even small savings at an early age translates to huge amounts of money. Start saving for retirement as soon as you get your first job.
Everybody needs to understand Algebra at a minimum so they can identify the necessary amount of savings, and solve to find the rate of savings and required rate of return needed so that they can hit that goal. Everybody else will be end up old and poor.