Riots, Drills and the Devil: Part 2 - S1-E7
Corrected entry: Michael sees that Tancredi needs help in the sickroom/nurse room from a TV monitor. So he climbs up to the roof and looks down to see Tancredi screaming for help. So he goes down, under the field. And then he is in the space between the inner roof and inner ceiling, and he walks to save Tancredi. Why didn't they use this escape way when the first failed? And they didn't need that key from Tancredi. They could escape just by walking in the ceiling space.
Correction: The crawl space lead to the sick bay in B wing and not the infirmary.