Their grades are ... than ours.

Their grades are better than ours. Их оценки превосходнее наших.

Their grades are worse than ours.  Их оценки ужаснее наших.

Their grades are higher than ours. Их оценки выше наших.

Their grades are lower than ours.  Их оценки ниже наших.

Their grades are nicer than ours. Их оценки лучше, чем наши.

Their grades are not better than ours.

Their grades are not worse than ours.

Their grades are not higher than ours.

Their grades are not lower than ours.

Their grades are not nicer than ours.

Are their grades better than ours?

Are their grades worse than ours?

Are their grades higher than ours?

Are their grades lower than ours?

Are their grades nicer than ours?