How in the name of arse do these characters manage to buy drinks in bars and clubs? They're sixteen. Or possibly seventeen. Still — either way, it's totally illegal and would never happen.
Likely through fake IDs, and sometimes through smooth-talking the guards (for getting into clubs) or bartenders - we see Effy do this to the guard at a club in her S2 episode, which leads him to let her and Pandora in and keep Tony out.
Is this a serious question (honestly, I can't tell with that last sentence)? Even here in America, where our drinking age is three years higher (thus making it harder for your average teen to pass as old-enough) and the police are much more concerned about underage drinking, plenty of teens are still able to get alcohol with fake IDs. I imagine it's even easier in Britain. It's not rocket science, at least if you know the right people to get a convincing fake... which, considering all the drug-dealing the Skins characters are involved with, they probably do.
There are always bars/clubs that are more lax on the enforcement thing also. A lot (most?) of teenagers do drink in real life.
Where does Cassie get all this money, as a runaway, to just randomly fly to the USA? And how does she get a job as a waitress as a non-citizen without so much as a green card?
As to the second part - very, very easily, especially given that waitresses are overwhelmingly paid in cash and can be considered part-time employees. American attitudes towards illegal immigrants is a little more complicated than Arizona would have you believe.
I know that last bit, I'm an American myself. But I've applied for jobs like waitressing, and even those jobs, even part-time, require legal documentation that you're eligible to work in the United States. She's not just being paid in cash tips; there would have to be some sort of legal red-tape she'd have to wade through to get that job, unless they're paying her under-the-table.
To answer the first part, a lot of the characters sell drugs at clubs and parties to get cash for themselves. This is established in Series 1, Episode 1, and pretty much rings true throughout the show. Cassie definitely does this, as she also has access to pills that she probably doesn't take.
But as that same episode establishes, most of that money would go back to pay their dealers. In Skins, selling drugs is a way to earn spending money, not a way to earn a living wage.
How does Sid know that Jake is fifteen?
The fact that Grace never really woke up from her coma has some unfortunate implications: firstly that the tender intercourse scene towards the end of Rich's S6 episode was really just him on his own masturbating, and even worse, that he just imagined Grace telling him to f*** her in the hospital and what Mr. Blood walked in on was Rich raping her comatose body. With that in mind, Rich got off extremely (and unrealistically) lightly by only being kicked out of the hospital! It should have been a police matter that would most likely end in rich being sent off to the loony house! WTF?
Letting that pretty extreme issue slide, it seems odd that Blood finally accepts that Grace really did love Rich and vice versa at the end, and lets Rich comfort him, but then seems to be back to his old ways in the next episode when he doesn't let anyone come to the funeral.
The way I see it he always knew she loved him he just didn’t care because he felt that she was pissing her life away even if she did. That scene of him a Rich together was suppose to be Blood saying “OK I don’t approve of you and I never would have, but now she’d dead and I know you where the only boy she ever loved so I think it would be a betrayal of her not to share this with you”, don’t know why he didn’t invite her other friends to the funeral
We don't even know if Rich was at the funeral, do we? We didn't see him complaining about not being invited with the others but only because he disappeared completely for a few episodes afterwards. And if Blood genuinely cared about not betraying her memory, he would invite the whole gang because he had to know deep down that Grace would have wanted them there. The only reason he ever had to dislike them was that HE thought they would be bad for her in the long-term, but now that she's dead anyway, all that should matter is that they loved her, and should get to say goodbye to her. Still, I guess you'll only get so far trying to apply logic to him, he was a Jerkass from the start. Also, Rich rapist issue still unresolved.