Reinhard's reaction was to keep out of the battle, his own well being was the most important thing to him after all. However, he also couldn't just let two students hurt, if not kill each other with noble arms involved, so he prepared himself to step in by encasing any wounded in a thin layer of pollen from his spear, which was leaning against the wall right next to the door of the classroom.
He slightly accelerated his breathing to prepare his circulatory system for the incoming load.

In the moment in which Pablo stepped in, there were only three way the situation could go, one of which Reinhard deemed very unlikely.
Most likely the combatants would ignore Pablo or start attacking him; considering Reinhard's past experiences here, it was unlikely the two would just stop.