This study focuses on the teachers' predictions of the students' performances -- in particular the middle-low achievers -- while solving tasks testing inquiry competencies. The tasks come from PISA science. More specifically we study science teachers' predictions for several aspects: levels of difficulty of the tasks, the potential sources of difficulty and the potential difficulty in solving it for medium-low achievers. We also study what assessed competencies are identified by science teachers in the tasks. Our approach is a questionnaire-based study. A sample of French teachers in science and technology (125) responded to the questionnaire. The teachers show a rather good ability to predict inquiry task levels of difficulty for medium-low achievers and are able to identify relevant potential sources of difficulty or easiness in the items. However, they are not aware of some essential difficulties that medium-low students encounter while solving science inquiry tasks. Moreover, the teachers have difficulty identifying the competencies that are tested by an item.