I would just tell him to explain to the kids that this is a shirt that someone gave him that he really likes because he really likes the person who gave it to him.
If the teasing only started today (yesterday) I don't think I'd write a note to the teacher. If it continues and it becomes obvious that it'll be an ongoing thing, I might just make her aware that it's causing problems.
For me, a lot would depend on whether or not you trust her (the teacher) to be observant or not - is she spacy- does she miss a lot of stuff? Or is she on top of things and maybe noticing, but trying to let Mo deal with it on his own before she jumps in? She may just be waiting to see and trying to let it die a quick death - if Mo doesn't react- they may just move onto someone or something else. The sad thing is - these type of kids always manage to find someone or something to make into their entertainment.
I'd talk to Mo and say, 'Look, these kids don't have anything better to do than look at your shirt...sad, isn't it? But you need to understand, if you're gonna keep wearing the shirt, you'll probably keep hearing it.'
Help him understand the cost of individualism.
My daughter had to wear hearing aids and one kid targeted her and said all sorts of stupid stuff and she never said a word until he finally said to her, 'Look, Olivia puts shrimps in her ears (they ARE kind of shaped like shrimps - I'd never noticed that before). She told me that one - and I told her that next time he said that she should ask him, 'Why do you think I'd put shrimps in my ears......that'd be pretty stupid....does your family put shrimp in your ears or something?'
I don't know if she ever did, but she realized that people who make stupid suggestions like that are the ones who are silly - not the people who are just going about making their own decisions and doing what they have to or want to do to get through the day.