如果你想根据每组中的 * 最高 * 分数来确定每组的分数范围，你可以这样写：

var ordered = students.OrderByDescending(s => s.Marks).ToList();
int groupId = 0;
int nextIndex = 0;
while (nextIndex < ordered.Count)
{
    groupId++;
    var firstInGroup = ordered[nextIndex];
    var group = ordered.Skip(nextIndex).
                        TakeWhile(s => firstInGroup.Marks - s.Marks <= 5).ToList();
    group.ForEach(s => s.GroupID = groupId);
    nextIndex += group.Count;
}

要使此方法可重用并允许在任一方向上进行分组，可以将其转换为helper方法，如下所示：

enum MarkGroupingOption { HighestToLowest, LowestToHighest }

static void SetGroupIds(List<Students> students, int marksThreshold, 
                        MarkGroupingOption groupingOption)
{
    var ordered = (groupingOption == MarkGroupingOption.HighestToLowest
        ? students.OrderByDescending(s => s.Marks).ToList()
        : students.OrderBy(s => s.Marks).ToList());

    int groupId = 0;
    int nextIndex = 0;
    while (nextIndex < ordered.Count)
    {
        groupId++;
        var firstInGroup = ordered[nextIndex];
        var remaining = ordered.Skip(nextIndex);
        var group = (groupingOption == MarkGroupingOption.HighestToLowest
            ? remaining.TakeWhile(s => firstInGroup.Marks - s.Marks <= marksThreshold)
            : remaining.TakeWhile(s => s.Marks - firstInGroup.Marks <= marksThreshold)
            ).ToList();
        group.ForEach(s => s.GroupID = groupId);
        nextIndex += group.Count;
    }
}

用法：

SetGroupIds(students, 5, MarkGroupingOption.HighestToLowest);

SetGroupIds(students, 5, MarkGroupingOption.LowestToHighest);