Jott, a messaging app that works without a data plan or WiFi connection, has caught on among junior high and high school students, according to co-founder Jared Allgood. He says the app more than doubled to half a million active users in March, up from 150,000 active users previous. Allgood told TechCrunch that the app continues to gain momentum, adding 15,000 to 20,000 users a day. That’s consistent with numbers from App Annie. The app started ranking steadily in the top 75 on iOS for social networking in the U.S. in mid-April. The reason? Teens who don’t have a data plan that will allow them to text are using their iPods and iPads to message each other on a closed network within a 100-foot area within school limits.