And it could be... that's why he wanted you to call it in. He was worried too. Glad you told him no, though! You're absolutely right that they would ask things only he could answer, during the report. Do you have "no loitering" signs up? Just wondering if this might help. At least it would be a public notice of your intent - then, when the report goes in - at least it'll be harder to brush off the report. Police are stretched pretty thin, these days, I think.
Teenagers have hung out like that, at least since I was a teen - usually it's just an annoyance; an irritation to the neighborhood. But if they are chased out - maybe they'd end up somewhere they really have no business being, ya know? Sometimes, it's possible to try talking to them - find out a little about them - in a sincerely nice way. I'm gonna guess they don't have anything else to do and no one at home cares. Too bad they'd probably refuse an offer of helping out around for there, for what y'all could afford to pay them. (That's my Pollyanna perspective.)
On the other hand (and way more realistically speaking) - I know full well the less than cordial (!) and intimidating stance of a group of teenaged boys (and these days, girls too) who feel like any normal human, neighborly interaction somehow has a nefarious, evil ulterior motive. Groups who are just a moment away from pretending to be - or being - thugs. And I completely understand how you really have to cautious... and that even the appearance of becoming a regular "hangout" for the kids quickly turns to something worse.
SIGH. Even in my day it was hard for kids who felt "in the way" and unwanted and uncool at home to find something productive to do. But at least we still had strong neighborhoods. The kids were organized from time to time, to donate time/skills/grunt labor to helping out older folks who couldn't tackle those jobs themselves. We picked up trash along back roads outside of town and around the parks we liked to hang out in. We volunteered - I was a candy striper for awhile. Why couldn't kids who need parental attention and guidance be paired up with seniors who physically can't do what they used to - to each other's mutual benefit?? Or am I just living in a fuzzy dream about the "good old days"?