If you get caught kissing at school whether it be by an authoritative figure or a fellow classmate the best thing you can do is to simply own up to it. You performed the act of kissing and you were caught. If you get into trouble it likely will not be much trouble so do what is asked of you. The best thing to do is be honest with yourself and with others about what you just did.
First rule, don't kiss at school! It's kind of gross to subject others to your wild hormones (So perhaps extend this to don't get all smoochy smoochy in public. A peck is fine, but anything else is vulgar). Second rule, don't deny. Tell the truth and you're likely to get a less harsh punishment.
What CAN you do about it? Kissing is normal, though a make-out session at school is inappropriate. Show whomever you're in trouble with that you're still focused on your studies and don't sweat it. One day you'll look back and it will seem ridiculous that you even worried about this.
I'm in eight grade. So me and my girlfriend got caught making out NEXT TO the locker rooms and the gymmteacher, Mrs. McGibbon, caught us and said "Coming up for air soon, or even better, class?". It was so embarrassing, and as soon as she left, we continued. BAD MISTAKE!! She came back and we had to talk to the vice principal about it. She let us off with a warning. Bad idea...