Okay, compulsory British schooling goes from 5-16 but you can choose to stay on until 18 in a "Sixth form". The first year is called reception (similar to kindergarten I think) and goes from ages 4-5, then year 1 is from 5-6, year 2 from 6-7 and so on. Years R to 2 are at infant school, years 3-6 are junior and years 7-11 are secondary (although this sometimes is different.) You can leave at the end of year 11. (This is when you take O Levels) O Levels are now called GCSEs (General Certificate of Secondary Education) and you can score A* to G as a pass and then U or ungraded as a fail, although most schools consider Cs to be passes. A GCSE course usually lasts about 2 years. 'A' Level courses are taken in the 6th form and also last 2 years. The first years you take AS levels (half of an A Level) and the second year you complete the whole A Level. Most people usually take 8-10 GCSEs and 3 or 4 A Levels.
A public school is in fact a private school! You have to pay for them and sometimes I think you have to take a test to get in. State schools are free (apart from taxes) and in theory, accept children of all abilities. I hope this helps! Anythign else you want to know, just ask!
The legal drinking age is 18 and the age of consent is 16 for heterosexuals and 18 for homosexuals although I think there are currently debates about changing this.