Politeness levels of speech and their selection are just as strongly associated with social situation or occasion as they are with the relative social standings of the people present...if not even more so.
Just because the teacher occupies a higher status than the students does not dictate that the teacher use plain/impolite speech forms.
Note that the teacher may (or may not) drop his speech level when talking to any of the students in a different setting.
Speech level "code switching" is a common thing in Japanese speaking.