My school grades have risen quite a bit above average.
Literal
I [of-の] school [of-の] grades [topic-は] average [object-を] considerably above-came.
上回る ('to exceed, to go above') takes を for the thing exceeded — a marked pattern where the thing surpassed is the direct object. ~てきた = 'have come to [verb],' expressing a gradual change up to the present. かなり ('considerably, quite') is a strong intensifier.