how far does one scold kids...

Sigh, I hate scolding students. and it doesn't help when they are defensive and make me think I accused them wrongly. It makes me feel very bad for the rest of the day. I've always thought I'm pretty good with disciplining children...but correcting youths is so much more complicated and tricky. Sometimes I don't really know what to say to convince them.  It's so difficult not to lose my temper when I'm actually really short-tempered.

Anyone has any suggestions? :P