I would say when I was at high school, it was considered generally normal teasing, even when it crossed the line into sexual harassment. Me and another girl in my form both had red hair, and both were regularly asked about the colour of our pubic hair, sometimes in quite intimate detail. Teachers definitely witnessed it, but none ever stepped in. It was stuff that really wasn't ok to be said to 11-13 year old girls as well as the usual teasing type stuff.
However, I would say the level of teasing is probably related to attractiveness- I had a relatively early puberty, and by 15 I was pretty "well developed" and as my attractiveness increased the teasing/bullying basically stopped. However, that doesn't make it right, and I do think it is something schools should take more seriously. I learned to give as good as I got, but that doesn't make it ok. I was very insecure about my looks as a teenager (more so than usual), and I do think the teasing about my hair continued.
I don't think it's the same as racism, but I do think it has some roots in anti-irish sentiments. There's also seems to be some anti-redhead sentiments in the catholic church, and I have heard that redheads were more likely to be burned as witches (don't know how true it is though).
However, it is actually a superior mutation, at least in the UK- the pale skin and lack of pigmentation enables us to absorb more vitamin D, meaning redheads historically were less likely to get rickets and other diseases.
Basically, all bullying needs to be treated seriously, but I do think there are specific examples with the teasing of redheads (particularly when it becomes sexual in nature) when schools/adults should step in.