Eu entendi issomensagem de erro:

The type arguments for method 'System.Linq.Enumerable.Select<TSource,TResult>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,int,TResult>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

O primeiro método não tem problemas ao usar umIEnumerable<T>.Select() ? Onde está o problema com o segundo método?

private void GetPupilsForSchoolclass()
{
   ObservableCollection<PupilViewModel> pupilsOC = new ObservableCollection<PupilViewModel>
   (                            _adminRepo.GetPupilsBySchoolclassId(_selectedSchoolclass.SchoolclassId).Select(p => new       PupilViewModel(p, _adminRepo))
   );
   SelectedSchoolclass.PupilListViewModel = pupilsOC;
}

private void GetDocumentsForPupil()
{
                ObservableCollection<Document> documentsOC = new ObservableCollection<Document>();
                IEnumerable<Document> documents = _docRepo.GetDocumentsByPupilId(_selectedPupil.Id);
                documents.Select(doc => documentsOC.Add(doc));
                SelectedPupil.Documents.DocumentList = documentsOC;
}