Between school and my job, I don't have much time for anything else.
The phrase means "given the combined (encroachments or demands of) school and my job" there is no time left over for other activities.
So, in your example, it would be "given the combined (encroachments or demands of) Neville and Ron" a good two-minute conversation with Cho was unlikely. The nature of those encroachments or demands is supplied by context. For example, it could be that the two of them, whether individually or as a duo, were monopolizing Cho's time, or monopolizing Harry's time, or that they were goofing off with each other so that there were constant hijinks and never a quiet moment where Harry could talk with Cho. Whatever. You'd have to look to the wider context in the book to get a sense of how Ron and Neville were interfering in this.