These beetles were swarming on an outside wall at Wettingel School.
It is a beetle in Tenebrionidae family, the darkling beetles, most probably Gonocephalum adpressiforme.
These beetles are very common on Guam. They are scavangers found in leaf litter and they don't cause any damage.
Click here to see an image of Gonocephalum adpressiforme on Gerald McCormack's Cook Island's Biodiversity site.
This incident resulted in a couple of news stories: