The elementary school that I work at has a population that is highly impacted by trauma, and we are working on becoming a “trauma informed school” through lots of professional development. You nailed it — you can’t tell which kids are dealing with trauma, but you can respond to children in a way that won’t re-traumatize them and can help them cope.
Poor Frank sounds like he’s struggling with a lot of inner demons without having the language or skills to express them appropriately. That sounds like a really tough situation to deal with for the after-school program. It’s probable that he’s coming to you guys after similarly long, hard days at school too. I’m sure that somebody at his school must be taking some steps to address his needs — I wonder if contacting the school to talk about what strategies/tools are used there would be beneficial? It seems like setting up as much consistency as possible between his different environments would be really helpful for a kid like Frank — as well as being around people like you who will not give up on him.