Not all online competitions promoted to schools are worth entering. Where contestants are really thin on the ground, schools can find their sub-par entry actually winning a prize, and then being promoted worldwide by the competition's sponsor as an example of the school's excellence.
A site still under construction assigned to a pupil at a girls' school in north London recently won a prize in a computers and writing competition sponsored by US publishing comany McGraw-Hill. Anyone visiting the Writing Room on the winning site (obscurely hosted by servers at Reading University), learns that "the Writing Room is being refurbished". In other words, it has yet to materialise.
To gauge the value of competitions promoted largely online, teachers should first check previous winning entries. For competitions in their first year, evaluating sponsors, or asking colleagues' advice, are worthwhile safety checks.