What do casting directors look for in teen actors?
Training is number one, especially with teens, because as mentioned in the interview below with a casting director, teen actors have to be able to deliver on a show, which means having strong comedic and dramatic acting skills and an acting technique they can rely on.
The other important thing mentioned in the video is that casting directors look for teen actors who are actively pursuing acting careers, so if you want to act as a teen, you have to keep at it and audition regularly. This shows that you are serious about your career. Casting directors often don't cast actors the first time they audition, but if they like them, they will call them back for the next project they have.
Finally, singing and dancing training can be very helpful for teen actors, as well as any other skills like playing an instrument or excelling in sports. Make sure you mention it all on the special skills section on your resume.
Finally, just like adult actors, teen actors have to think about type. Are you a lead type, a best friend/girl-next-door type, a character type (high school nerd, rebel, etc.)?