var query = schüler
    .Where(s => s.Name == "Fritz")
    .Select(s => new 
    {
        Lehrer  = s.Lehrer, 
        Schüler = s.Name
    });

internal class Program {
  private static void Main(string[] args) {

    Schueler s1 = new Schueler() {id = 1001, name = "Fritz"};
    Schueler s2 = new Schueler() {id = 1002, name = "Maria"};
    Schueler s3 = new Schueler() {id = 1003, name = "Klaus"};

    List<Lehrer> ll = new List<Lehrer>();
    ll.Add(new Lehrer() {id = 1, name = "Lehrer 1", schueler = new List<Schueler>() {s1, s2, s3}});
    ll.Add(new Lehrer() {id = 2, name = "Lehrer 2", schueler = new List<Schueler>() {s1, s3}});
    ll.Add(new Lehrer() {id = 3, name = "Lehrer 3", schueler = new List<Schueler>() {s2, s3}});

    var x = ll.Select(l => {
                        Lehrer nl = l;
                        nl.schueler = l.schueler.Where(s => s.name == "Fritz");
                        return !nl.schueler.Any() ? null : nl;
                      });

    foreach (Lehrer x1 in x) {
      Console.WriteLine(x1.name);
      foreach (Schueler schueler in x1.schueler) {
        Console.WriteLine("  " + schueler.name);
      }
    }
    Console.WriteLine("---");
    Console.ReadKey();
  }

}

internal class Lehrer : Person {
  public IEnumerable<Schueler> schueler = new List<Schueler>();
}

internal class Schueler : Person {}

internal abstract class Person {
  public int id;
  public string name;
}