The vent was quite up in the wall that the zombies could not reach that high.
All 3 of the boys jumped inside the vent and that vent was big enough for all of them to fit in.
Raiden and Toshi were totally exhausted due to all the running. They constantly run up the stairs for about 7 or so floors. They laid low in that vent and gasped for air.
The other boy was on the edge of the vent. He wanted to reach the other side but Toshi and Raiden blocked his way. He did not interrupt them as they gasped for air as he himself knew how much they had to constantly run to reach the top.
They stayed in the vent for some time.
The zombies were scratching on the wall below the vent. They knew where their meal was but could not reach it. All the zombies stayed put in that room. The number of zombies was so big that they even filled the corridor outside of the room.
Raiden and Toshi were still rapidly gasping. But, the boy was done waiting.