Get unique value list

this.schoolYearArray = [...new Set(this.attendanceDetails.map(item => item.SchoolYear))];

Groupby list

groupBy(list, keyGetter) {
  const map = new Map();
  list.forEach((item) => {
    const key = keyGetter(item);
    const collection = map.get(key);
    if (!collection) {
      map.set(key, [item]);
    } else {
      collection.push(item);
    }
  });
  return map;
}

implementation:

const grouped = this.groupBy(this.attendanceDetails, attendanceDetail => attendanceDetail.SchoolYear);