My DS is that position in quite a few subjects dream - his coursework is B grade, or thereabouts, but he's got his act together lately and is now getting A or even A in practice exams, but would be unlikely to get an A overall because the courswork will pull him down. Might get an A though, as the coursework is mostly only 25% or so of the final mark. It's a bit of a shame when he really can work at an A/A* level now, but the courswork was done a year or more ago sometimes when his writing skills were poorer and he wasn't very good at figuring out what examiners were looking for.
second - is it a state school? If it is I'd assume they simply mean "wear a shirt and tie" when they say 'black tie'. Still yet to broach this issue with DS who seems convinced he can turn up in tracksuit bottoms..... Am hoping his friends will inform him otherwise so I don't have to. His mates aren't exactly the most switched on to dress sense though, to put it mildly 