If you are planning to visit or attend a school in the U.S., no matter for how long or whether you will get credits, you will need to apply for a F-1 student visa, or, if accepted into an organized exchange program, a J-1 visa. See our website for more information.
F-1 students are prohibited from attending public elementary and middle schools (up to and including grade 8) or attending a public high school for more than one year. Transfer of Guardianship/Custodianship to someone in the U.S., or admittance to the school, does not change this rule. Information on visa requirements and how to organize a privately arranged stay at a school in the U.S. is available at http://educationusa.de/fileadmin/inhalte/pdf/High_School-Jahr_in_den_USA__Version_1.3.pdf (PDF, German) und http://travel.state.gov/content/visas/english/study-exchange/student/foreign-students-in-public-schools.html.
Exception to the rule: Short term stays as part of an exchange between schools aimed at furthering cultural understanding may be possible without a visa, provided that one is not actually enrolled at the American school in question nor receives academic credit nor spends more than 18 hours per week in class. Please contact us if you have questions.