It's beneficial for string players (and all young musicians) to experience music-making and performing via as many outlets as possible. Our local school systems offer amazing Regional, District, and All-State Orchestra Events every year. Auditions are typically at the beginning of the school year for events that take place January-March. Students can talk to their school teachers about obtaining audition materials. Fairfax County VBODA offers the Solo & Ensemble Festival for students to perform solo or in small ensembles and receive feedback from a judge on their playing. It's an excellent opportunity and a lot of fun, too! I am busy helping many of my students prepare for their respective Festivals -- coming up shortly!
Lastly, there are some excellent youth orchestra organizations in the D.C. area. The youth orchestras typically rehearse once per week and present concerts 3-4 times throughout the school year. Admittance to these orchestras is typically based on an audition scheduled in May or June of the prior school year. It's enriching, a great way to meet fellow string players from other schools, and improves musicianship. A couple of organizations I recommend:
Washington Metropolitan Orchestras - An excellent organization that rehearses locally in the Alexandria area
American Youth Philharmonic Orchestras - A very well-respected set of orchestras which has opportunities for a wide swath of ages and abilities
Youth Orchestras of Prince William - Another great organization - I am an alumna of these orchestras and have fond memories of fun concerts and their gifted conductors and section coaches