To draw mouths in comics well, you need to understand facial expressions. Observe how the mouth changes in different emotions. Also, practice different mouth shapes and styles to get better at it.
Teenagers are in a stage of growth and development, and we should focus on positive topics related to their growth, such as their dreams, hobbies, and achievements in study and art. Telling stories about their physical features in a so - called'sexy' way is inappropriate and may violate ethical norms.
A story about a magician who could fit many objects in his mouth during his magic shows. He trained his jaws and mouth muscles to be able to do this amazing feat.
Start with simple shapes like ovals and curves. Pay attention to the position and size of the lips and teeth. Experiment with different expressions to add character.
You can start by making the corners of the mouth downturned sharply. Also, add some sharp lines to show tension. And don't forget to shade it to give it a more intense look.