Index: iknow-api/Services/Implementations/ProfService.cs
===================================================================
--- iknow-api/Services/Implementations/ProfService.cs	(revision ea405569ef29f56fbfc4d77708b6b3faf9286097)
+++ iknow-api/Services/Implementations/ProfService.cs	(revision ea405569ef29f56fbfc4d77708b6b3faf9286097)
@@ -0,0 +1,122 @@
+using iknow_api.DTOs;
+using iknow_api.Models;
+using iknow_api.Repositories;
+
+namespace iknow_api.Services
+{
+    public class ProfService : IProfService
+    {
+        private readonly IProfRepository _profRepository;
+
+        public ProfService(IProfRepository profRepository)
+        {
+            _profRepository = profRepository;
+        }
+
+        public async Task<List<SubjectStudentsDto>> GetStudentsBySubjectAsync(int professorId)
+        {
+            var taught = await _profRepository.GetTaughtSemesterSubjectsAsync(professorId);
+
+            return taught
+                .Where(ss => ss.Subject != null && ss.EnrolledSemester?.User != null)
+                .GroupBy(ss => ss.Subject!.Id)
+                .Select(group =>
+                {
+                    var subject = group.First().Subject!;
+                    return new SubjectStudentsDto
+                    {
+                        Id = subject.Id,
+                        Name = subject.Name,
+                        Code = subject.Code,
+                        Users = group
+                            .Select(ss =>
+                            {
+                                var student = ss.EnrolledSemester!.User!;
+                                var semester = ss.EnrolledSemester.Semester;
+                                return new EnrolledStudentDto
+                                {
+                                    Id = student.Id,
+                                    Name = $"{student.Name} {student.Surname}".Trim(),
+                                    Index = student.Index,
+                                    // 0 means "not graded yet"; the enum starts at 6.
+                                    Grade = ss.PassedSubject is null ? 0 : (int)ss.PassedSubject.Grade,
+                                    Semester = semester is null
+                                        ? null
+                                        : $"{(semester.Type == sType.winter ? "Зимски" : "Летен")}({semester.Year}/{semester.Year + 1})"
+                                };
+                            })
+                            .OrderBy(u => u.Name)
+                            .ToList()
+                    };
+                })
+                .OrderBy(s => s.Name)
+                .ToList();
+        }
+
+        public async Task<GradeResultDto> SetGradeAsync(
+            int professorId, GradeRequestDto request, GradeAction action)
+        {
+            // grade_type only declares 6..10, so anything else cannot be stored.
+            if (action != GradeAction.Remove && !Enum.IsDefined(typeof(Grade), request.Grade))
+            {
+                return Fail($"Grade must be between 6 and 10; got {request.Grade}.");
+            }
+
+            // This also enforces authorisation: the lookup is scoped to the
+            // subjects this professor actually teaches.
+            var semesterSubject = await _profRepository.FindTaughtSemesterSubjectAsync(
+                professorId, request.StudentId, request.SubjectId);
+
+            if (semesterSubject is null)
+            {
+                return Fail("You do not teach this student in this subject.");
+            }
+
+            var existing = await _profRepository.GetPassedSubjectAsync(semesterSubject.Id);
+
+            switch (action)
+            {
+                case GradeAction.Add:
+                    if (existing is not null)
+                    {
+                        return Fail("This subject is already graded. Use edit to change the grade.");
+                    }
+                    await _profRepository.AddPassedSubjectAsync(new PassedSubject
+                    {
+                        SemesterSubjectId = semesterSubject.Id,
+                        Grade = (Grade)request.Grade,
+                        // date_passed is `timestamp without time zone`.
+                        DatePassed = DateTime.SpecifyKind(DateTime.UtcNow, DateTimeKind.Unspecified)
+                    });
+                    return Ok($"Grade {request.Grade} added.");
+
+                case GradeAction.Edit:
+                    if (existing is null)
+                    {
+                        return Fail("This subject has no grade yet. Use add first.");
+                    }
+                    existing.Grade = (Grade)request.Grade;
+                    existing.DatePassed = DateTime.SpecifyKind(DateTime.UtcNow, DateTimeKind.Unspecified);
+                    await _profRepository.UpdatePassedSubjectAsync(existing);
+                    return Ok($"Grade changed to {request.Grade}.");
+
+                case GradeAction.Remove:
+                    if (existing is null)
+                    {
+                        return Fail("This subject has no grade to remove.");
+                    }
+                    await _profRepository.RemovePassedSubjectAsync(existing);
+                    return Ok("Grade removed.");
+
+                default:
+                    return Fail("Unknown action.");
+            }
+        }
+
+        private static GradeResultDto Ok(string message) =>
+            new() { Ok = true, Message = message };
+
+        private static GradeResultDto Fail(string message) =>
+            new() { Ok = false, Message = message };
+    }
+}