I know you try to better your students’ spoken English, but don’t interrupt their discourses: they’re communicating with one another! And this already is a nice step.
You can be writing down or scribbling some mistakes or errors, and comment on them, but I’d tell you that you should proceed into communication: you shall correct afterward.
The same happens in written tests: first have into account that tentatively they, the students are trying just to say something – so remember, not much red ink.
Do correct some mistakes and errors anyway, I would say. Even you can write short comments in English on the back of the test papers, stimulating, encouraging, explaining why some stuff is wrong, why some stuff is right. / Photo from: benuambassador wordpress com