‘Even though some of the students had receipts showing they had paid for their meals’
either there are very stupid cops involved here, or there’s an element to this story we’re not being told...
The key word is “some”.
It’s possible that five of the ten had paid, but the other five had not, or seven had paid, five of which had receipts, but three had not.
Having been young myself in the area around the University of Washington, I can see a large group doing this. And so can the cops. That’s why they rounded up the whole group.