LibreOffice homework. Resolving redundancy. "translating" two foreign keys (either of which is absent) into one (supertype)

I am using LibreOffice Base with embedded HSQLDB for educational purposes.

The subject area is MOOCs. I have the following entities: Student, Teacher, User(superclass), Course, Forum, Question, Progress Journal.

For Course I want to distinguish whether it is Student or Teacher, Students study many courses (n:m) and Teachers teach many (n:m); but for Question, in my case, I do not need any distinction - one User (either Student OR Teacher) can ask many questions on the Forum (1:n).

Here is the picture of my Relations Table.

Please, ignore the intended redundancy of Student/Teacher attributes, I was required to have 6 entities at least.

Obviously, the User cannot have both ID_Student and ID_Teacher as non-empty, either of them should be Nan.

How do I solve redundancy of free space (Nan-s), or should I leave it as is?

Or maybe the problem lies in my peculiar definition of the subject area?

I could have created the ID_user attribute in Question which would have been storing both the Student (1:n) and Teacher (1:n) IDs, given that their IDs don't collide. I was unsure whether it was correct, so created separate User entity.

When trying to use REFERENCES keyword I face an error: Unexpected token: REFERENCES in statement [create table students ( student_id int primary key references]

Is there another way around? Should I download any extras to make LibreOffice execute it properly?

The foreign keys are the wrong way around.

User is the supertype, so student and teacher should have foreign keys to that:

create table users ( user_id int primary key, ... );

create table students ( student_id int primary key references users ( user_id ), ... );

create table users ( teacher_id int primary key references users ( user_id ), ... );