There are quite few boarding schools that offer summer credit course for US history. You can check each boarding school website to find out. The best way for you is to use google or go into the website of boarding school:TABS to find the list of boarding schools. That is a start.
However, before enrolling summer course, one important thing to remember is that a student should consult with the school whether or not the school will accept the credit for US history course that student will take during summer. Often the current school does not like this idea of finishing early.
