下面是一个示例代码，演示如何按条件进行减少和分组：

# 假设有一个包含学生信息的列表
students = [
    {"name": "Alice", "grade": "A", "score": 90},
    {"name": "Bob", "grade": "B", "score": 80},
    {"name": "Charlie", "grade": "A", "score": 95},
    {"name": "David", "grade": "C", "score": 70},
    {"name": "Eva", "grade": "B", "score": 85},
    {"name": "Frank", "grade": "C", "score": 75}
]

# 按成绩分组，并计算每个分组的平均分
grade_group = {}
for student in students:
    grade = student["grade"]
    score = student["score"]
    if grade not in grade_group:
        grade_group[grade] = {"total_score": score, "count": 1}
    else:
        grade_group[grade]["total_score"] += score
        grade_group[grade]["count"] += 1

for grade, data in grade_group.items():
    avg_score = data["total_score"] / data["count"]
    print(f"Average score for grade {grade}: {avg_score}")

# 按条件进行减少
filtered_students = [student for student in students if student["score"] >= 85]
print("Students with score >= 85:")
for student in filtered_students:
    print(student["name"])

这个示例代码中，首先按照成绩分组，并计算每个分组的总分和人数。然后，根据条件（分数大于等于85）进行筛选，得到符合条件的学生列表。