你好！ 你美国成绩如果还不错的话，春季寄宿学校10年级下申请成功的几率比较大。我建议你去考一个托福成绩，不用很高，70多分就有用处。

想转的话， 最好有托福成绩， 另外有些学校也会安排10年级考psat, 这个成绩有些学校也是可以接受的。

Since you are already in a school in the US, I believe that your English grade is sufficient and you don't need to take the TOEFL. That is unless your English grade is not good then you might have to consider doing that. As for the SSAT, some schools will want it but some might waive that. When my daughter transferred from PEA to Milton, she didn't have to take the SSAT again since she already took one the year before.
Your chances are much better transferring from a US school to another US school. But the percentage depends on which school you are applying to.