Your school system should be able to advise you on the necessity of waivers. At most schools there will already be sports programs and procedures in place. Get your form approved by the principal before it is sent home. Make sure to let parents know about the benefits of the program, logistics, any fees to attend, time and place, contact people, etc. Structure your sessions.
Kids are comfortable with structure.
First, design a general structure for each time you meet. Here's an example for an after school program. Check in with leader. Put belongings in certain spot. Check to see that shoes are tied.