"up to scratch" is a suitable idiom meaning not bad, but not particularly good.
Other terms that can be used to mean "勉 強 可 以" :
just adequate
acceptable
satisfactory
just up to standard
barely meet the requirement
例 如 形 容 d 食 物
除 左 可 以 話 " The food is not bad" 之 外 , 可 以講:
The food is up to scratch.