Refusing could annoy the boy, and judging by the other teens looking at you, he didn't choose that place at random. And he seems nice, he shouldn't be mean if you're not. Furthermore, you just rule in maths, and if he tries to become friends with you (or more), he might be complimenting you! If you do it well, you could do two birds one stone.
"Yes of course!"
"Thanks"
He seems happy and sits shyly. But you notice that the real danger seems the teens looking at you. They could say anything, you need to solve this problem. And they seems amused that you accept the guy's request.
"I'm Josef, by the way."
"I'm.. Sarah, nice to meet you. "
Fuck, that's true, you can't even say your name anymore!
"Hey do you know who are those guys looking at us?"
"They're just my friends, don't pay attention to them."