Good coaches appreciate being thanked at the end of the season if they have done a good job. My son Hunter's freshman soccer coach, Mr. Fuery, did a fabulous job. I made a point of sending him a letter praising his fairness (he gave every player roughly equal playing time), quality teaching skills and knowledge about child development. Hunter had grown several inches during the spring and summer prior to the season and was going through an "awkward stage" so typical of adolescent boys. I sent a copy of my letter to the athletic director, principal and school committee.
On the other end of the spectrum was Spencer's freshman lacrosse coach, who had made life miserable for Spencer and his teammates by his constant yelling. I also sent him a letter (with copies to the AD, principal and school committee, and other parents), although that letter was not to thank him, of course, but to tell him how badly he had treated Spencer and the rest of the team (always try to use language like, "I am concerned for all the kids") in the vain hope that he might take my constructive criticism to heart (fortunately for the lacrosse players who followed Spencer, the coach "retired" before the beginning of the next season).
If your criticisms are shared by the other parents, ask them to send similar letters to the coach, with copies to the AD. By being proactive, you just might save the kids who are a year behind yours from a bad experience.