All equipment (where applicable): instrument/sticks/reeds/music/flags/poles/rifles/etc.
Music - always. Yes, this is listed twice. Always have your music and drill on you at all times even, if you have it memorized.
Pencils/highlighters for marking music
Water bottle/canteen for on-field practices
Bring a packed lunch
Eat breakfast before morning rehearsals
Comfortable and weather appropriate clothes. See school guidelines on dress code, check the weather, and plan accordingly.
Appropriate footwear: athletic socks and gym shoes - no flip flops, sandals, crocks, or dress shoes!
Hat(s) to block sun exposure (wind players especially should wear one to avoid sunburned lips which will affect your playing)
Sunglasses and sunscreen
Snacks - Any food items should be sent in re-sealable plastic containers or Ziploc bags. Food, other than water, is prohibited on the field.