[JacobAsmuth]

That would make sense if the author specified a specific year's "year group" - e.g. if they had said "I spoke to the 6th year group". But they said they spoke to "THE year group" - which is very confusing to me, even more so after your explanation.

[jimnotgym]

I think he is just trying to say he spoke to 'a' year group. If he said he spoke to a class, would that help? You are right that it is not that specific, a year group maybe one class, multiple classes or part of a class.