Although I can see why he does it, it's still a little painful how the author keeps using the "tournament interruption" card to give the main characters legitimate reasons to be the ones actually fighting instead of the teachers. Get the main characters in the arena (tournament or duel), invader appears, automatic shutdown happens and now the teachers can't help so the first year students have to fight. The gospel incident is the same where Houki (speed) and Ichika (oneshot) are the only ones equipped appropriately for the mission so the teachers can't go.
It's a good way to avoid the facepalming situation of the time Laura went berserk yet they let Ichika (who has low shield energy and is just a first year) handle the situation instead of the teachers. I actually sympathized with Houki when she gave the brilliant idea of letting the teachers handle it but then the show acted as if what Ichika was doing was the right thing (unless of course the novels explained why it had to be a low energy Ichika). However, using interruptions this often is hurting the story as many have pointed out.
Things I can think of that the author can use to change the routine:
Also, I guess another reason why the invader fights have to occur in the arena is because Tabane (as crazy as she is) probably isn't that willing to let civilians get hurt in her training routine/data gathering so she sets it up only on occassions where the arena's shields are active and when the main characters are in position (like in tournaments, duels or in the middle of the ocean).