I have worked with school groups (Booster Clubs, PTO, ect) and it is hard to get into, but not impossible.
AS Flitterfly said - ALWAYS deliver what and when you promise. Make sure you completely understand what it is they are looking for (both in the type of clothing and design). An actual sample of the product is the best selling tool as the customer will know exactly what they are getting. I always provide samples of the product in all of the sizes that it is available in so that the customer can select the correct sizes for everyone on the team or participating in the event and not just guess.
If you concern yourself more with satisfying your customer than in how much money you are making from them, things will work out better.
Fix any problems immediately. Unless it is going to cost a lot of money, I fix any errors at no charge even if it was not my mistake. Most of the people involved are volunteers and they are donating their time to get the clothing for their sports team or school event, etc. If there is an unresolved problem, they will pass this along to the next one to volunteer and you may lose a $2,000 job next year because you did not want to pay out an additional $50 this year to replace a jacket.
On the other hand, because many of these school organizations work with people they know and the people in charge change every year, you can provide excellent service and products in a timely manner and still lose that group for the following year. Again, don't take it personally. Try again the next year.
If you keep customer satisfaction as your number one priority, you will gain a good reputation and this is the best selling tool for the long term success of your business.