What you're describing doesn't sound like "padding" to me. If you plan on continuing tennis at prep school, you should absolutely include it in your application, no matter how recently you started it (this goes for any club). If you're concerned about them knowing why you started so recently, you can easily explain that to an interviewer (the interviews are there for you to tell them things that there aren't spaces to talk about on the application). And even though you won't be in the same position in terms of recruitment as someone who has been playing tennis since they could walk, remember that for someone your age, two years of a sport is longer than you think. Many kids start new sports in middle school. You don't need to have taken lessons your whole life for them to take you seriously as a player.