Getting “type or namespace name could not be found” but everything seems ok?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;

namespace DataTableExample
{
    class Program
    {
        static void Main(string[] args)
        {

            DataTable dt = new DataTable("Student");

            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("Branch", typeof(string));
            dt.Columns.Add("Age", typeof(int));
            dt.Columns.Add("SocialMarks", typeof(int));
            dt.Columns.Add("MathsMarks", typeof(int));
            dt.Columns.Add("ScienceMarks", typeof(int));
            dt.Columns.Add("TotalMarks", typeof(int));
            dt.Columns.Add("AverageMarks", typeof(decimal));


            List<Students> students = new List<Students>();//Students the type or namespace coulnt be found 

            Console.WriteLine("Please enter a number to create students and start processing");
            string[] classes = { "Semester-I", "Semester-II", "Semester-III", "Semester-IV" };
            string[] Branches = { "Computers", "Electronics", "Civil", "Mechanics" };



            Random cls = new Random();
            Random brnc = new Random();
            Random ag = new Random();
            Random marks1 = new Random();
            Random marks2 = new Random();
            Random marks3 = new Random();
            int n = int.Parse(Console.ReadLine());
            for (int i = 0; i < n; i++)
            {



                DataRow dr = dt.NewRow();
                dr["id"] = i;
                dr["Name"] = "Name" + i.ToString();
                dr["Branch"] = Branches[brnc.Next(0, 3)];
                int age = ag.Next(17, 22);
                int socialMarks = marks1.Next(50, 100);
                int mathsMarks = marks2.Next(50, 100);
                int scienceMarks = marks3.Next(50, 100);
                int totalMarks = socialMarks + mathsMarks + scienceMarks;
                dr["Age"] = age;
                dr["SocialMarks"] = socialMarks;
                dr["MathsMarks"] = mathsMarks;
                dr["ScienceMarks"] = scienceMarks;
                dr["TotalMarks"] = totalMarks;
                dr["AverageMarks"] = (float.Parse(dr["TotalMarks"].ToString()) / 3).ToString();
                dt.Rows.Add(dr);
            }
            Console.WriteLine("from data table");
            Console.WriteLine("ID | Name | Age| Branch | Socail | Science | Maths | Total Marks|Average");
            foreach (DataRow dr in dt.Rows)

            {
                Console.WriteLine(dr["Id"] + "| " + dr["Name"] + "|" + dr["Age"] + "|" + dr["Branch"] + "|" + dr["SocialMarks"] + "|" + dr["ScienceMarks"] + "|" + dr["MathsMarks"] + "|" + dr["TotalMarks"] + "|" + dr["AverageMarks"]);
            }
       

            Console.WriteLine("list the count of students agewise");
            var groupbyage = Student.GroupBy(s => s.Age).Select(lst => new { Age = lst.Key, Count = lst.Count() });
            foreach (var ags in groupbyage.OrderBy(lst => lst.Age))
            {
                Console.WriteLine("Age : " + ags.Age + " | Count: " + ags.Count);
            }