diff --git a/frontend/src/OnlineClassroom/ClassroomUserCreation.tsx b/frontend/src/OnlineClassroom/ClassroomUserCreation.tsx
index f2fa364..d32da0f 100644
--- a/frontend/src/OnlineClassroom/ClassroomUserCreation.tsx
+++ b/frontend/src/OnlineClassroom/ClassroomUserCreation.tsx
@@ -31,7 +31,17 @@ export function ClassroomUserCreation(props: {
         const paternalSurname = props.person.person_paternal_surname;
         const maternalSurname = props.person.person_maternal_surname;
 
-        const user = `${firstName[0]}${secondName?.[0] ?? ""}${paternalSurname}${maternalSurname[0]}`;
+        // The username may be composed of A-Z & 0-9.
+        const user = `${firstName[0]}${secondName?.[0] ?? ""}${paternalSurname}${maternalSurname[0]}`
+            .toUpperCase()
+            .replaceAll("Á", "A")
+            .replaceAll("É", "E")
+            .replaceAll("Í", "I")
+            .replaceAll("Ó", "O")
+            .replaceAll("Ú", "U")
+            .replaceAll("Ñ", "N")
+            .replaceAll(" ", "")
+            .replaceAll("-", "");
 
         const next_date = new Date();
         next_date.setDate(next_date.getDate() + 60);