Index: src/lib/prof-demo-store.ts
===================================================================
--- src/lib/prof-demo-store.ts	(revision 298f9b3987050da08cb65ce62c3d1f8295faeff4)
+++ src/lib/prof-demo-store.ts	(revision 298f9b3987050da08cb65ce62c3d1f8295faeff4)
@@ -0,0 +1,107 @@
+export type UsersBySubject = {
+  Id?: string;
+  Name?: string;
+  Grade: number;
+};
+
+export type SubjectsAndUsers = {
+  Name?: string;
+  Id?: number;
+  Users: UsersBySubject[];
+};
+
+export type AddGrade = {
+  StudentId: number;
+  SubjectId: number;
+  grade: number;
+};
+
+type Store = {
+  subjects: SubjectsAndUsers[];
+};
+
+function createInitialStore(): Store {
+  return {
+    subjects: [
+      {
+        Id: 101,
+        Name: "Web Programming",
+        Users: [
+          { Id: "20001", Name: "Ana Petrovska", Grade: 0 },
+          { Id: "20002", Name: "Marko Trajkov", Grade: 8 },
+          { Id: "20003", Name: "Elena Stojanova", Grade: 0 },
+        ],
+      },
+      {
+        Id: 102,
+        Name: "Databases",
+        Users: [
+          { Id: "20001", Name: "Ana Petrovska", Grade: 0 },
+          { Id: "20004", Name: "Nikola Iliev", Grade: 9 },
+          { Id: "20005", Name: "Sara Dimitrova", Grade: 7 },
+        ],
+      },
+      {
+        Id: 103,
+        Name: "Algorithms",
+        Users: [
+          { Id: "20002", Name: "Marko Trajkov", Grade: 0 },
+          { Id: "20006", Name: "Ivana Kostova", Grade: 10 },
+        ],
+      },
+    ],
+  };
+}
+
+function getGlobalStore(): Store {
+  const globalKey = "__iknow_prof_demo_store__";
+  const globalObj = globalThis as unknown as Record<string, unknown>;
+
+  if (!globalObj[globalKey]) {
+    globalObj[globalKey] = createInitialStore();
+  }
+
+  return globalObj[globalKey] as Store;
+}
+
+export function getSubjects(): SubjectsAndUsers[] {
+  return getGlobalStore().subjects;
+}
+
+function parseStudentId(id: string | undefined): number | null {
+  if (!id) return null;
+  const parsed = Number.parseInt(id, 10);
+  return Number.isFinite(parsed) ? parsed : null;
+}
+
+export function setGrade(
+  payload: AddGrade,
+  mode: "add" | "edit" | "remove",
+): { subject: SubjectsAndUsers; user: UsersBySubject } {
+  const store = getGlobalStore();
+  const subject = store.subjects.find((s) => s.Id === payload.SubjectId);
+  if (!subject) {
+    throw new Error("Subject not found");
+  }
+
+  const user = subject.Users.find((u) => parseStudentId(u.Id) === payload.StudentId);
+  if (!user) {
+    throw new Error("Student not found in subject");
+  }
+
+  if (mode === "remove") {
+    user.Grade = 0;
+    return { subject, user };
+  }
+
+  if (payload.grade < 5 || payload.grade > 10) {
+    throw new Error("Grade must be between 5 and 10");
+  }
+
+  if (mode === "add" && user.Grade > 0) {
+    throw new Error("Grade already exists; use edit");
+  }
+
+  user.Grade = payload.grade;
+  return { subject, user };
+}