Hi, I'm from england so can fill you in !
The grade system at GCSE level works from U as upgradable being the lowest, F, E , D ,C ,B ,A, and finally A* being the highest grade possible, most A* are achieved for getting above 90%.
Days usually start between 8-9 in the morning, with some sort of registration period at the beginning, or a break time, which is usaully around 10:30. The day has lunch around 12:30, with the end of the day being around 3:30 to 4:15, however once in college this becomes later as twilight periods are usually introduced.
College is years 12 and 13, and in the UK you get A levels which enable you to get into a university, like i guess in the US to study a degree.
Times for school and college opening and closure can vary in private schools and religious schools, and they may be required to attend on saturdays.
Hope this helps and good luck ! :)