O.W.L.s and N.E.W.T.s

[jrwilkens]

No: HPFGUIDX 34536

As an American (well out of high school!) I was curious if anyone familiar with education in Britain could 
tell me if there are anything similar to these delightfully named tests taken by Hogwarts students in the 
British system?  In other words, standarized tests taken around age 15 as well as in the last school 
year?  

Also what are the twelve possible O.W.L.s one can achieve?  (I'm assuming twelve is maximum based on 
quotes relating to Percy and Barty Crouch, etal.)  Are passing these tests necessary to graduation and if 
so, does passing at least one O.W.L. count?

JRW (who is terribly obsessive-compulsive about tests and likes to know exactly where she stands on 
them!)