let input = [
  {
    id: 1, val: '学校', parentid: null
  },
  {    
    id: 2, val: '班级1',  parentid: 1
  },
  {
    id: 3, val: '班级2', parentid: 1
  },
  {
    id: 4, val: '学生1', parentid: 2
  },
  {
    id:5, val: '学生2', parentid: 3
  },
  {
      id: 6,val: '学生3', parentid: 3
  }
]

let output = {
  id: 1,
  val: '学校',
  children: [
    {
      id: 2, val: '班级1',
      children: [
        {
          id: 4, val: '学生1',
          children: []
        },
        {
          id:5, val: '学生2', 
          children: []
        }
      ]
    },
    {
      id: 3, val: '班级2',
      children: [
        {
          id: 6,val: '学生3', 
          children: []
        }
      ]
    }
  ]
}

// 代码实现

function arrayToTree(array){
  // 依次处理
  let root = array[0]
  array.shift()
  let tree = {
    id: root.id,
    val: root.val,
    children: array.length > 0 ? toTree(root.id, array): []
  }
  return tree
}

function toTree(parentid, array){
  let children = []
  let len = array.length
  for (let i = 0; i < len; i++) {
    let node = array[i]
    if(node.parentid === parentid){
      children.push({
        id: node.id,
        val: node.val,
        // 递归
        children: toTree(node.id,array)
      })
    }
  }
  return children
}

console.log(arrayToTree(input))