Great care has been taken in the design of Story mode to make the mathematical difficulty increase at just the right rate for almost all pupils.
We should all anticipate a certain amount of "struggle" to pass some levels - that's the nature of learning and improving, of course - so it's appropriate that pupils take between 1 and 5 attempts to pass a level, on average.
Teachers can check a pupil's average attempts to pass a level by going to Stats > Story Record. If you receive feedback that a pupil is having significant difficulty, take a look at what the data is telling you. Have they made many attempts to pass the current level? Is their average-attempts-to-pass looking high?
There will be individual pupils, however, for whom the time constraints required to pass a Story level are demonstrably too tough for them.
In those one or two cases, having considered other options such as gentle encouragement, offline help from a grown-up, etc., the most sensible way forward is to relax the time constraints somewhat for them.
Two and Three Times Longer
Teachers can give certain pupils two or three times longer to answer each question by going to the Story Controls page and moving the pupils' nametags into a group then setting that group on "2× longer" or "3× longer".