It is important that much time is spent training in the ring in preparation for kumite. Working only in a class situation and expecting to do well at a karate tournament is about as realistic as training to be a distance runner and expecting to be able to do well as a sprinter. Tournament karate is a sport like any other and the training must match the actual conditions as closely as possible for the training to be effective. Practice rings can be set up in the dojo and you can simulate actual matches. Opponents you fight against should be as diverse as possible. You should fight opponents of different body sizes, speeds, and abilities. Each person fights differently with different strengths and weaknesses so it is important to fight as many different people as possible. It is also important to train and fight those that are better in ability than yourself on a regular basis. If you constantly dominate your training partner, it will be much different when you face off with an actual tournament opponent that is much better than you. Fighting and training with someone better than you will ultimately make you a better fighter. The better fighter will make your weaknesses known to you and give you an opportunity to work on making those vulnerabilities stronger. The better fighter will also push you to the limits of your abilities and help you to extend those limits and in the process make you stronger.
The most realistic training in the ring is fighting in tournaments themselves. Nothing pumps the adrenaline and simulates the real thing like an actual tournament. Competing in small regional tournaments will give you invaluable in-ring experience that will prove most useful in bigger more important tournaments. Again it is a case of practicing what you desire to excel in.