HINT: This is supposed to be funny!
"Thank you for coming," said the head, "You will see there are several lipstick kisses in the mirrors in this washroom.."
Some of the girls grinned at each other.
"As you will understand, modern lipstick is cleverly designed to stay on the lips, and so the lipstick is not easy at all to clean from the mirrors. We have therefore had to develop a special cleaning regime, and my hope is that when you see the effort involved you will help spread the word that we'd all be better off if those responsible for the kisses use tissue paper instead of the mirrors in future.."
At this point the caretaker stepped forward with a sponge squeegee, which he took into one of the toilet cubicles, dipped into the toilet bowl, and then used to clean one of the lipstick-covered mirrors.
The caretaker smiled. The girls departed. And there were no more lipstick kisses on the mirrors.