What should I do if the teacher sees me kissing my girlfriend in the classroom?

Generally speaking, the school won't let you go to high school because of this. If your teacher asks, you don't have to be afraid. You said that two people just touched their heads with something, giving people the feeling that they were kissing, but nothing happened. Besides, everything needs to be proved now. You can be criticized just because your classmates complain, and you can tell the teacher that you don't have it. If you ask the teacher to produce evidence, such as photos or videos, you can say that I am kissing someone just because my classmate said so. As long as you are justified, there will be no big problem unless the teacher sees it with his own eyes.