Personally, I think it's silly as hell to care too much how a kindergardener styles his hair.
That said, this is a charter school with uniform and grooming rules. My sons also attend a charter school with a uniform and grooming rules. From the article, it sounds like these parents were warned three times that the child's hair was not in line with the school's standards.
They chose to ignore these warnings, disrespect the school's policies and do whatever they wanted - essentially, they set their child up to be expelled and I find that unacceptable. I don't respect the parents's choices here and I think they were in the wrong.