What should I do if a boy with fair skin and little body hair is laughed at by his classmates, especially because his butt looks like that. Girl is very troubled...

In northern my country (Inner Mongolia, Shanxi and other provinces), such people are not uncommon. They belong to the race of immigrants from Western nations. They used to be called "color-eyed people". They have fair skin and yellowish hair and pupils. It is quite different from the Central Plains people with dark eyes and yellow skin. No need to worry about this. This trouble is a common phenomenon among boys entering puberty.