This has been our experience too! Dreading next year a little bit but certainly spending a lot of time with the school calendar at the moment. The issues are random events that crop up and changes in trips etc that have been a particular bug bear. Not sure how to completely avoid this though. We have arranged to have parent meetings for each of the groups doing controlled assessments (ICT and Computing) to inform them of what exactly is involved and to try and get their support but this is by no means a solution.