I have a couple students who will be entering high school this fall, and they got me thinking about what students (who play any instrument, not just flute) should know to be well prepared for high school band or orchestra programs.
1.) Be able to play all 2-octave major and minor scales from memory. District and allstate auditions usually require some version of this, so you'll be ahead of the curve if you already have your scales memorized.
2.) Know the difference between flat, sharp, and in tune, as well as how to tell if you're in tune and how to fix it if you aren't. A few minutes every day with a tuner will give you a much better awareness of whether you're playing in tune or not.
3.) Have good hand position. If you don't have a private teacher to help you figure out how to hold your instrument with a hand position that's as ergonomic as possible, look online for examples. Be watchful about the health of your wrists, arms, shoulders, and back; it's hard to undo damage from an overuse injury.
4.) Get in the habit of purchasing copies of the solo pieces you work on. Working from photocopies is questionably legal at best, and you can't always get a legible version from online sources. Also, walking into an audition with a photocopy is highly frowned upon unless the organization you're auditioning for specifically provided a photocopy for your use. Imslp.org is a great resource if you just want to check out a bunch of different public domain pieces while deciding on one to learn, but once you choose one, it's time to buy a hard copy.
Spend some time this summer developing these skills and habits, and you'll be the leader of the pack when high school starts in the fall!