A teacher collection which wraps the logic of operations on teachers. To support getting the index, the GetIndex method is introduced and the collection is ordered by the creation date by default on the constructor. This demonstrates a design issue.

TeacherCollection.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace DotConnect.LeakyAbstraction
{
    public class TeacherCollection : IEnumerable<Teacher>
    {
        private readonly IList<Teacher> _teachers = new List<Teacher>();

        public TeacherCollection(IEnumerable<Teacher> teachers)
        {
            if (teachers != null)
                _teachers = teachers.Where(x => x.IsStillAtWork)
                .OrderBy(x => x.StartedOn)
                .ToList();
        }

        public TeacherCollection WhereTeachMathematics()
        {
            return new TeacherCollection(_teachers.Where(x => x.Specialty == "Mathematics"));
        }

        public TeacherCollection WhereExperienced()
        {
            return new TeacherCollection(_teachers.Where(x => DateTime.Now >= x.StartedOn.AddYears(10)));
        }

        public IEnumerator<Teacher> GetEnumerator()
        {
            return _teachers.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return _teachers.GetEnumerator();
        }

        public int Count
        {
            get { return _teachers.Count; }
        }

        public int GetIndex(string teacherName)
        {
            for(var index = 0; index < _teachers.Count; index ++)
            {
                if(_teachers[index].Name == teacherName)
                {
                    return index;
                }
            }
            return -1;
        }
    }
}