Since it was after school, there were only a handful of students on the sports ground.
Literal
After-school was [because-ので], sports-ground [at/topic-には] mere slight students [only-しか] did-not-exist
しか + negative is the 'only' construction: 生徒しかいなかった = 'there were only students (and not more).' ほんのわずかな layers two intensifiers of smallness — ほんの ('mere') + わずかな ('slight/few'). The explanatory ので links the cause to the observed result.