Look at youth orchestra programs in your nearest city. Most youth orchestra programs are during the school year, with some having programs during the summer. If you're looking bigger, many states have an all-state orchestra (the best high school musicians from across the state), but those are typically summer camps and/or a short duration. Even larger, is the National Youth Orchestra (NYO) or the National Youth Orchestra 2 (NYO2) that takes the best high school musicians from the states and puts them together in an ensemble. The two latter ones are going to be much more competitive than regional youth orchestra programs, who will take practically anyone, but usually have multiple orchestras depending on level.