It's okay to challenge students. In fact its essential to learning. If a task is set at the correct level of challenge then 50% of pupils should fail at it. That’s potentially a lot of failure for pupils to deal with everyday, but as a school and as a classroom practitioner, we need to support that failure and show pupils the link between failure and learning. The overall outcome of the process is what pupils are eventually judged on through assessments, but pupils need to understand that the process they go through to understand a new concept is just as important (if not more important) as getting it right.