Adding More Stories During The Sprint To Keep The Developers Busy

Hi my name is Joshua and I work as a Scrum Master in a company providing IT solutions to production companies. My team has a habit of adding on more stories in the sprint just so the team members remain occupied. The team includes 1 senior developer and 4 junior developers. The senior develop is familiar with the project and does the product review. The current practise is the team selects some stories for development and starts developing them through the sprints. The senior developer takes up complex stories requiring more development time while the junior developers take up less important ones. As a result, when the sprint is half way through, the tasks taken up by the junior developers gets completed and they don't have further stories to develop. The senior developer, on the other hand, remains occupied with complex stories throughout the sprint. The team starts pulling in new stories as and when the junior developers complete them. The review backlog keeps on enlarging since the senior developer does not have the time to review them. As a Scrum Master, I put a ban to this practice. I explained to the developers it is important for them to focus upon the development work already taken and complete it properly before taking up additional stories. This has led to discussions with the CTO who insist that the developers are wasting their time doing nothing during the latter part of the sprint and they should have the right to take up stories to increase productivity. How should I tackle this situation? Please advise!!

Posted By - ross