回复：亲戚孩子想来美国读高中

1.美国只有私立高中发I-20,公立是不发I-20的是吗?

yes

私立高中的I-20好申请吗?

depends on school. good school=difficult; bad school=easy

需要些什么材料?

depends on school. go to their websites or call them

2.读高中需要过语言考试吗(比如TOFEL或雅思)?

depends on schools. mostly ISEE or OLSAT; good schools also require interview normally. contact the school directly

3.美国这边需要出担保人吗?

not necessary

4.读高中的所有费用,包括TUITION,校外住宿生活费用等(我想美国高中很少有寄宿的吧)

depends on school. Good boarding schools normally > $40,000 a year.

no boarding normally $20,000 a year for good schools.

5.国内有什么中介公司可以办理这种全套出国手续或有什么网站可以介绍这方面的信息?

don't know about 国内 :-)