the support for TurtleBot-2 robot in ROS Melodic is not fine yet AFAIK (https://answers.ros.org/question/294600/ros-melodic-does-it-support-turtlebot2/). And some of the exercises of JdeRobot Academy target to this robot, so we stick to currently working underlying infrastructure: Ubuntu 16.04 +ROS Kinetic + Gazebo7 (this Gazebo release is well tested to work with ROS Kinetic).
It is planned a migration to ROS Melodic and Gazebo 9 (and many other newer packages) soon, in the beginning of 2019. The idea is to simultaneously upgrade to more recent releases of many underlying infrastructure, always sticking to official packages (not maintained by us, at least in ROS, Gazebo, OpenCV…). Some testing with them is required before adopting them as the infrastructure of Academy. This way the installation keeps simple as the user only has to install official packages.
In addition, we are in the process of releasing a Docker image with all the underlying infrastructure already installed and ready to use in JdeRobot-Academy.
I would suggest go on with Ubuntu-16.04+ROS Kinetic+Gazebo 7