That his marks fell, on average, between fair (not bad) and disappointing - i.e. that most of them were either fair or disappointing. The phrase [adj] to [adj] is quite common in English and comes from the world of horse racing where the condition of the turf can be described as "good to middling" or "fair to good" to mean that there is no overall way of summarising the whole situation. "The weather was fair to good when I was on holiday" - some days it was fair, some days it was good.

"Его оценки за выпускные экзамены справедливо разочаровали"...либо, могу предположить, что "его справедливые оценки за выпускные экзамены разочаровали".....