Is it grammatically correct to except something without it being part of the original group? [closed]

In Arabic, it is allowed to say for instance:

All the students left the room except the teacher.

So, the teacher is put after an "except" even though he is not one of the students. It's probably a bit of an obscure usage even in Arabic.

Does it exist in English in some obscure rule or usage?

Except is used to denote exclusion from a group where something would otherwise be included. If something is not a member of the original group, you should not use "except" to exclude it.

In the example sentence, since the teacher is not a student, you cannot use "except" to exclude them from a group of students. The sentence as written implies that the teacher themselves is a student, which is likely not the intended meaning. To use "except", you must use a grouping that would normally include the teacher, such as:

Everybody left the room except the teacher.