First things first take a deep breath and calm down.
Second yes and no.I know confusing right?Well if your will is strong and you seriously hate that person wishing ill on them may not always work because and said given moment you might have concocted a million and one ways for said person to suffer.AS such your magic may not work because your head(thiughts and emotions) are all over the place.
Thirdly if your mildly stressed/irratated it means you can focus on something a single particulae thought.You could just be wishing for some irratating person to shut it and in would walk the teacher soon after,or some likely situation like that.
Fourthly don't worry about it if you have the mind set that something bad is going to happen it will.The threefold law is there for a reason.It is my belief(may not be others) that the three fold law only affects those who intentionally do something wrong and not who accidentally did something.Now lets say for arguments sake cause I'm sure someone will point this out it does not matter,I say it matters if it was unknowingly done you wont have anything to worry about the universe will have mercy on you for you are only human.
Dont frett for your life because everything will be fine.Be positive.Wear a good luck charm and one for protection if you wish.Learn to shield and all will be.