Mastery System needs to be reworked
There is no reason why I can go 12/0/1, get Tower first blood, and have 75 cs before the enemy surrenders at 16 minutes and still only get an A+.
Honestly, a successful surrender should increase the opposing team's mastery grade by 1 step for each person at or over that threshold (i.e. that A+ would be S- with 4v1 surrender and S with unanimous).