This may be tough to understand but there’s a ton of still completely-intact, dense urban environment in between the border and the school in your scenario. So on your way to the school, you can be ambushed.

Or hey, maybe thinking strategically even a little bit, maybe the school, or the road, or any of the surroundings are booby-trapped! Maybe they’ve planted IEDs or mines on the path there!

It’s very far fetched, I know. Humans haven’t been waging war for forever, there isn’t a giant history to pull from.

“The terrorists are using schools as shields though!”

“Oh damn that’s a genius strategy. Better just give up every military advantage I have and send in my soldiers to be ambushed.”