To know what to say at the end of a karate class or dojo you should learn the karate etiquette or martial arts rules and How does one greet a karate master?
Normally you would shake hands and introduce yourself. If you are in your karate uniform a bow is appropriate. We typically follow that with a hand shake as well, as Miyagi teaches the karate kid in the movie "karate kid"

Entering the dojo The first lesson you will ever learn once you have been accepted into any karate school is how to enter and exit the "dojo" or "training hall" properly. Prior to entering the dojo for the first time a senior student or “Sempai” will usually instruct you in the art of "bowing in". Every karate dojo in the world has a shrine at the designated “front” of the dojo, this is referred to as the "Shomen" and regardless of how many times you enter or leave the dojo during the course of your daily training, you must always bow to the "Shomen" first. This is done by standing at the dojo entrance and facing towards the "Shomen", be sure that your feet are together, keep your legs straight, your arms should be at your sides and touching the sides of your thighs, your hands should be open and facing downward along the seam of your gi with your fingers and thumb together. To bow, bend forward at the waist to about 45 degrees, keep your eyes looking downward and do not let your arms move or leave your side, pause for a second at the bottom of the bow then unbend. The entire bow should take only a few seconds, but it should be performed with the utmost courtesy and respect.
during class Once the training starts it is very important to put aside all unrelated thoughts. You must make every effort to only concentrate on the specific task at hand and especially on improving the quality of your own techniques. On "seeing" what can not be seen. In the beginning this will be a very hard concept for you to grasp but you must push yourself both physical as well as mentally if your karate is ever going to improve. In short, "always do your best". In fact to do otherwise would be disrespectful not only to yourself, but also to your classmates and your instructor who have come to train with you.
The end of class Often this comes all to soon. When your class is at an end and “Line Up!” is called once again, be sure and quickly line up in in the same manner and rank order as you were at the start of your class. Finish as you started, with a positive attitude and a willing desire to always do your best no matter what lies ahead. It is very important that the lessons you learn at each class leave the dojo with you. How you use and apply these lessons in your everyday life is up to you, but your progress depends on you remembering them and building upon them. Upon instruction come to attention, and then bow, after the final bow to the Shomen and the sensei students will often say “arigato gozaimashita", which means “thank you very much” or simply say “thank you” in English in either case it is the level of gratitude is important. In truth "class" never ends.
The end If you keep these simple rules in mind then your experiences within the dojo will be much more enjoyable, and your fellow students will hold you in much higher regard for your efforts.
Remember
A good effort is the result of
having the proper mental attitude.
Part the clouds - see the way.
"The objective of karate-do is to contribute to the evolution
of the human spirit through physical and mental training."
Sensei Peter Lindsay