In Harry Potter and the Order of the Phoenix, Harry attempts to use Umbridge's fire to contact Sirius after having his final vision during OWLs (Ordinary Wizarding Level exams; magical equivalent of high school diplomas I believe). When he's yanked back out he finds that five members of the DA ('Dumbledore's Army', commonly refereed to as DA in the books) have been captured and restrained by an equal number of Inquisitorial Squad members. It's made clear that the DA members have been roughed up, but no mention is made of any harm to the Inquisitorial Squad members.
I would concede that any duel could be won by circumstance and luck as much as skill. Still, it seems terribly unlikely that five separate duels are all lost by students who have spent the entire year preparing specifically for this eventuality. Further, so, that the winners should come out of it completely unscathed. This builds on the oddness of the earlier scene when the Inquisitorial Squad captures DA members fleeing from their final meeting, without any mention made of resistance (not even a shield charm).
While Umbridge may have given a slight advantage to the Inquisitorial Squad, she has been shown time and again to be incompetent, both in terms of general magical knowledge and in practical magical abilities. So it seems unlikely that she turned the tide so completely herself. And the DA members were engaged in an illegal activity and specifically on guard against Inquisitorial Squad members and Umbridge, making surprise unlikely. And it has been established that several members of the Inquisitorial Squad are not magically competent.
So how were the DA members so easily overpowered if, as the book claims earlier, they really were highly competent at defense against the dark arts? What is really going on here, that the DA members continually lose with minimal resistance to fellow students?