The t shirt has a skull and cross bones on it, not a slogan stating "I'm a little twat whose parents want to kick your fuckin head in"
If you are on a limited budget in a small town it is extremely difficult to get t shirts WISHOUT some sort of slogan on them.
As for excluding a four year old - well, you send babies to school, don't be surprised when they behave like babies.
Not all four year olds do as they are told. My ds1 couldn't have done it on pain of death. He couldn't do is on pain of severe punishment, as I found out when he was four. it is NOT necessarily the parents fault.
The light switch thing sounds exactly like ds1. He used to stand and fuck about with light switches too.