Jack knew straight away he shouldn't be here. He wasn't the type to do stupid things just because it might turn out fun and you might be fine, and maybe you might meet some interesting people. Well, he was sixteen now, starting his first year at Hogwarts, and he wanted to know all he could about, well, everything. That's the reason he was here.
Of course, having been only just introduced to the wizarding world, Jack didn't realise that in certain places, certain people had a certain status. Such as muggle borns, like him. But of course, he didn't know that.