The adverb makes 'he opened the door slowly' non-punctive. It is not clear whether 'looked' is describing a punctive act or a durative one.
If the former (the apparent tautology is idiomatic):
John opened the door slowly, taking a swift glance over his shoulder
as he did so. /
John took a swift glance over his shoulder as he slowly opened the
door.
If the latter:
John opened the door slowly, all the while looking back over his shoulder.
This answer refers to American English, I expect it's different in the UK (and perhaps from one UK country to another).
If someone is in elementary, middle, or high school, we usually ask
What grade are you in?
and the answer would either be a number grade First grade through Twelfth grade, or a high school student might answer with freshman, sophomore, junior, or senior (when I went to school 40 years ago, grades 7-9 were often junior high school, so freshmen didn't usually identify themselves that way).
If they're in college/university, the question is usually
What year are you in?
and the answer would be freshman, sophomore, junior, senior, or graduate school.
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I can recommend it, but purely for entertainment value.