AFS Intercultural Programs - Opportunities to spend a year of high school studying abroad or a shorter time period or even virtually in a variety of countries. Some offer need-based scholarships, others do not.
Benjamin Franklin Summer Institutes - in the US (North Carolina recently) with students from other countries, ages 16-18
Bureau of Exchange Programs - master list from the State Department, not limited to high school students
National Security Language Initiative for Youth (NSLI-Y) - programs to learn languages (e.g. Korean, Russian, Hindi, Bahasa, Arabic, Chinese, Persian/Tajiki, etc.) in a variety of countries around the world for ages 15-18 - deadline usually in November
Congress-Bundestag Youth Institute for High School Students - Spend a year at a German school, living with a host family - ages 15-18 (no German proficiency required)
Kennedy-Lugar Youth Exchange & Study (YES) Abroad - spend an academic year attending high school in a Muslim country (only Morocco and Senegal have a French language requirement) deadline in early December
Youth Ambassadors - 3-4 week exchange programs to Latin American and Caribbean countries
USA Jobs - have to create an account
Library by the People - Working with the Library of Congress - transcribing from cursive in English and Spanish
Smithsonian Digital Volunteers Transcription Center - women's history, science, African American History
Zooinverse - bunch of opportunities - observing science and transcribing including weather in Australia
Letters of Encouragement Campaign (MCPS .net emails needed)
Science Starter - including one to look at the shapes of cyclones!
iNaturalist - A community for naturalists
STEM SUMMER PROGRAMS
NIH - biomedical
NEI - Eye institute
NSA and even work study
Smithsonian - internships and programs
Courses - no need to take for credit, but if you can why not? Those in MC2 know of those options, those that don't there are plenty.
Others