"Wow, $100 per year is ludicrous. I spend more than that each year for my own personal art supplies. I agree with everyone else to gather the data about what you reasonably need to purchase each year. My budget is different now because I am at a private school, but when I taught at a public school I believe I had about $2000 (give or take a few hundred, can't remember) for about 150 kids? Then, when I taught at a middle school I had around the same amount $2000 for about 300 kids (although they rotated classes every quarter). Each of these budgets was adequate and covered the expenses for these kids. If you are truly stuck with this budget next year here are some tips for you.
-- set up an Artsonia.com account for your school. Each year our school earns a couple hundred dollars through that site.
-- join freecycle.org and you would be surprised at what people are offering for free... in just one week I saw posts for yarn, fabric scraps and scrap wood. You could also post specific requests for things you want. I asked for Sesame Street toys for my little girl and got 3 big bags of books, dvds, stuffed animals, a sprinkler.... people like to pass along their used items to people that would enjoy them. Plus, it's good for the environment. It's also a good way to get rid of strange things you don't need anymore.
-- Post things in your school newsletter and send out flyers for donations of art supplies that people don't use anymore.. you could ask for things like buttons, crayons, markers, beads, fabric, yarn... lots of people have lots of craft supplies they buy and never use. I even asked for aprons and one of our parents who owns a restaurant donated a class set of aprons!
-- When the kids have a supply list at the beginning of the year, see if you can add on some art supplies.. For example: All first graders bring in a box of markers and a glue stick which are gathered for general use in the art class. I have the students bring in pencil sharpeners, erasers, glue sticks, markers, black uniball pens and more. One year I had each child in one grade bring in a ream of computer paper and 3 years later, we still are making sketchbooks from that supply.
-- See if your community has a Hardware Re-store.. ours is a habitat for humanity one I believe. I have picked up ceramic floor tiles for pennies, wood boards, nails, all sorts of odds and ends for very cheap.
-- Check out thrift stores and garage sales. I have gotten so many good things from garage sales. Our local thrift stores have lots of hodge podgy craft items which you may be able to find use for.
-- Plead your case to your parent organization. I have gotten support in the past from the parents for special projects and supplies.
Good luck! $100 is a very tough budget!! Marcia