Ground nesting Bees in a school yard

Ground nesting bees usually emerge in June when soil temperatures increase from the cooler spring temperatures.  These bees are often more than a foot down in the ground.  Usually one bee uses one entrance, but in some species a few bees use the same entrance, but different nesting tunnels.

This aggregation is in a school yard ( Burnaby, BC).  When Alie told me about it, she showed me the area adjacent to a small fence.  the kids came out for a show and tell, and they were fascinated.  Anyone know the type of bee?

The soil is quite hard in this area and when the sun shines, the ground is always in the sun.  if you look closely, you can see little mounds of earth deposited by the emerging bees.  The scale becomes a little clearer when you see the children's feet in the area.  When all the children were back in their class, I checked out to see the scope of the area.  It was quite large  (10 x 10 m or 30 x 30 feet).