What are you so bummed about? B is Above Average! The vast majority of students only get 70% for hours of study. Take what you can get, talk to someone who got a higher grade, and learn their secret. I got Ds and Fs on a number of assignments and tests in school. Those grades told me that I wasn't cut out to be an engineer or a politician, or a lawyer. Bs and As in other subjects helped me discern that being a tutor was my calling.
Don't let one low grade (and 87% isn't low) ruin your drive. Everyone hits bumps, even the valedictorian. Figure out what you did wrong, and next time you will be ready for it. Take note of what you did well. If you can explain multiplication to a 3 year old, but Hamlet leaves you scratching your head, tutor math instead of Lit. If you can show a 6 year old the difference between an insect and an arachnid, but "Stopping by Woods on a Snowy Evening" just feels cold, tutor science instead of poetry.
Teaching (including tutoring) is one of the must underpaid, underappreciated careers in the world, but don't let that deter you. Teachers make all other career paths possible; remember that. Be glad you got 87 instead of 77, study harder next time to break into the 90s. You can do it, you just have to try.