Karellen is a free app offered by Karellen, an operating system for preschools. The app requires login credentials to access the school's data. It is designed to facilitate communication between parents and teachers. The app provides a calendar of the school and the weekly menu, along with pictures of the child's activities in school. The teacher can use the app to mark attendance and send direct messages to parents. Moreover, teachers can take pictures in the app and tag the names of the children appearing in each picture. Parents receive the picture immediately in their app. They can also mark their child as absent or sick the next day or coming days and send direct messages to the teachers. Karellen is an excellent tool for parents and teachers to stay connected and informed about the child's school activities.