If you already have something in mind, just gonna say my idea ((that I've had for some time now)):
So, like your idea, villains attack the bridge above the school, the teachers go check it out but don't come back, wich causes the students to worry and go see, i honestly think Zuron, Ciel, Asta and Jack would be the first to check, then the others come, there, they find their yeachers ((i changed it alot with your villains involved)) in barriers they can't destroy, so , everyone heads in to fight but gets defeated, later on, Jack makes up a plan to defeat and capture the barrier maker, explaining the plan to everyone, then as they try to put it in action, they are interrupted by Jack's brother wich attacks them all, in the end, ((insert a character)) ends up hitting the barrier maker , thus releasing the heroes and defeating them..
That was my idea....