I see it every year - at some point in the marching season, a clarinetist comes to me saying their instrument isn't working. A short investigation uncovers a crack in either the barrel or upper joint of the instrument, which leads to disappointment and sometimes tears. The barrel can be replaced, but a big enough crack in the joints means a new instrument. In either case, it means spending money when you didn't expect it. A crack in an instrument can be frustrating and expensive, but how can you keep it from happening?
Simple: don't march with a wooden clarinet! The weather conditions are so variable in the fall, and wood is sensitive to changes in temperature and humidity. Marching in 75 degree rainy weather one day and then doing a two-hour outdoor rehearsal in dry, 55 degree weather the next day is a formula for cracking. Wood instruments and marching band just don't mix.
But you need an instrument! Yes, and a plastic one is perfect. Check with relatives, friends, local music stores, and even yard sales, Craig's List, and eBay (please ask for help from your teacher when buying online!). You don't have to spend a lot of money, you just need a working instrument to play on the field. You should be able to find a suitable one for $100-$300. That little bit of investment can save you more expensive heartache later.