There are many things that you need to have in order to play an instrument. Some instruments need more equipment then others. Here is a list of mandatory supplies for each instrument.
Flute: C Flute, Tradition of Excellence Book 1 for flute, cleaning rod, and a swab to clean the instrument. You can make one out of a handkerchief. A wire/folding music stand for home practice and after-school rehearsals.
Clarinet: Bb Clarinet, a box of at least three strength # 2 Bb clarinet reeds (#2 strength are recommended for beginners; for returning 5th graders I recommend #2.5 strength. Returning 6th graders all should have ideally switched to #2.5 strength reeds already, but if not please consider switching to #2.5 the next time you buy reeds). Rico are ok and are the cheapest. Tradition of Excellence Book 1 for Bb clarinet. A wire/folding music stand for home practice and after-school rehearsals.
Alto Saxophone: Eb alto saxophone, a box of at least three strength # 2 reeds for alto saxophone (#2 strength are recommended for beginners; for returning 5th graders I recommend #2.5 strength. Returning 6th graders all should have ideally switched to #2.5 strength reeds already, but if not please consider switching to #2.5 the next time you buy reeds). A good neck strap with padding, a wire/folding music stand for home practice and after-school rehearsals, and Tradition of Excellence, Book 1 for Eb alto saxophone.
French Horn: Single horn in F or double horn in F/Bb, a wire/folding music stand for home practice and after-school rehearsals, rotary valve oil, and Tradition of Excellence, Book 1 for French Horn in F.
Trumpet: Bb trumpet, valve oil, and Tradition of Excellence, Book 1 for trumpet. A wire/folding music stand for home practice and after-school rehearsals.
Trombone: Tenor trombone, small water spray bottle, good trombone slide cream such as Slide-O-Mix, Trombotine, or Superslick (this is different from slide grease, which is for tuning slides and is also good to have) (I DO NOT RECOMMEND VALVE OIL), and Tradition of Excellence, Book 1 for trombone. A wire/folding music stand for home practice and after-school rehearsals.
Baritone: Baritone or Euphonium, valve oil, and Tradition of Excellence, Book 1 for baritone B.C.. There are two baritone books Treble Clef, T.C. and Bass Clef, B.C. Please make sure it is the B.C. book. A wire/folding music stand for home practice and after-school rehearsals.
Percussion: A percussion kit or its components (percussion kits typically include all of the below components, but sometimes may have only a snare pad instead of a snare pad and drum. I do not have a preference as to whether students use a snare pad or snare drum):
-Snare drum and/or snare pad and stand (please don't use a snare stand from a drumset as they are too short)
-A bells set/glockenspiel and its stand
-Mallets for the bells
-Snare drum sticks
-Tradition of Excellence, Book 1 for percussion. Make sure it has snare and mallets in the book. There are multiple types of percussion books out there, so be careful!
-A wire/folding music stand for home practice and after-school rehearsals.
Optional items:
Metronomes are very useful for all musicians, but it is mandatory for percussion since they have to keep a very strict steady beat. There are now several apps on mobile devices that can be downloaded for this.
Tuners: Good to have, but not a priority for beginners. There are now several apps on mobile devices that can be downloaded for this.
Reed Cases: These are very helpful for protecting reeds, but you can get by without one for now.
Instrument stand: If you would like one, but not mandatory. This is different from a music stand which is mandatory.
Cleaning Swabs: Clarinets, saxophones, and oboes all come with cheap swabs. You can buy silk swabs that work a lot better by the company "Gem."
Reeds (not optional for saxophones and clarinets, but the brand you choose is): I recommend Rico to start, but down the road you may want to consider buying a higher quality reed by Vandoren or even Rico Grand Concert or Rico Royal reeds.