Ladder Internships blurb:

Ladder Internships is a selective start-up internship program for high school students, founded and run by Harvard graduate students. In the program, students work 1-1 with supervisors from top start-ups to work on real-world projects in a real start-up environment. Our companies and supervisors come from Y Combinator, Harvard iLabs, Wharton, Google, Microsoft, and McKinsey & Company.

No prior working experience is required. The application deadline for the Spring cohort is February 18, here is the brochure and the application link.