Master Skinning (300-375) is learned in the outlands at your respective factions town (Alliance is Honor Hold, I don't have a hordie). For leveling quickly after 275 I recommend skinning runs in Sunken Temple, dragonkin there will get your skills up FAST.
BTW contrary to popular belief, you do NOT have to be level 50 to raise your skinning cap over 300. I had a guildmate port me to Shattrath at level 47 and then escort me to Honor Hold. Not sure if there IS a minimum level you must be to get Master skinning. I think as long as you are at 300 you can get it.