How do you get past the Prefects in the libary without being caught?

Try hitting them with flipendo. This usually stops them from casting Locomotor Mortis. Then keep running as fast as you can. If you hear locomotor mortis try casting flippendo again. You can also cast expelliarmus which stops the charm. If your trying to get past the prefects instead of running straight away watch the pattern they move in. Then use that patten to run to the other side. Good luck!

If you're close to one in front of you hit him with flippendo immediately and don't forget to use stink pellets when one is about to cast locomotomortis and experilliarmus helps because if one hits you with that spell, use expelliarmus with good timing and don't worry about fighting the prefects because the main goal is to run, run, AND run to the next door because if you open a door, you're safe.